MikeMcGrath

CodingforBeginners

IneasystepsisanimprintofInEasyStepsLimited

16HamiltonTerrace·HollyWalk·LeamingtonSpaWarwickshire·CV324LY

www.ineasysteps.com

Copyright©2015byInEasyStepsLimited.Allrightsreserved.Nopartofthisbookmaybereproducedortransmittedinanyformorbyanymeans,electronicormechanical,includingphotocopying,recording,orbyanyinformationstorageorretrievalsystem,withoutpriorwrittenpermissionfromthepublisher.

NoticeofLiability

Everyefforthasbeenmadetoensurethatthisbookcontainsaccurateandcurrentinformation.However,InEasyStepsLimitedandtheauthorshallnotbeliableforanylossordamagesufferedbyreadersasaresultofanyinformationcontainedherein.

Trademarks

Alltrademarksareacknowledgedasbelongingtotheirrespectivecompanies.

Contents1Gettingstarted

Programmingcode

Settingup

ExploringIDLE

Gettinghelp

Savingprograms

Storingvalues

Addingcomments

Namingrules

Summary

2SavingdataStoringinput

Controllingoutput

Recognizingtypes

Convertingdata

Guessinggame

Correctingerrors

Summary

3PerformingoperationsDoingarithmetic

Assigningvalues

Comparingvalues

Findingtruth

Testingcondition

Settingorder

Summary

4MakinglistsWritinglists

Changinglists

Fixinglists

Settinglists

Namingelements

Summary

5ControllingblocksBranchingchoices

Countingloops

Loopingconditions

Skippingloops

Catchingerrors

Summary

6CreatingfunctionsDefiningblocks

Addingparameters

Returningresults

Storingfunctions

Importingfunctions

Summary

7SortingalgorithmsCopyingsorts

Selectingsorts

Insertingsorts

Bubblingsorts

Mergingsorts

Partitioningsorts

Summary

8ImportinglibrariesInspectingPython

Doingmathematics

Calculatingdecimals

Tellingtime

Runningtimers

Summary

9ManagingtextManipulatingstrings

Formattingstrings

Modifyingstrings

Accessingfiles

Manipulatingcontent

Updatingcontent

Summary

10ProgrammingobjectsDefiningclasses

Copyinginstances

Addressingproperties

Derivingclasses

Overridingmethods

Applyingsense

Summary

11BuildinginterfacesLaunchinginterfaces

Respondingbuttons

Displayingmessages

Gatheringentries

Listingoptions

Pollingradios

Checkingboxes

Addingimages

Summary

12DevelopingappsGeneratingrandoms

Planningneeds

Designinglayout

Assigningstatics

Loadingdynamics

Addingfunctionality

Testingprograms

Deployingapplications

Summary

13TransferringskillsUnderstandingcompilers

Compilingcode

CodingC

CodingC++

CodingC#

CodingJava

Summary

PrefaceThecreationofthisbookhasprovidedme,MikeMcGrath,awelcomeopportunitytoproduceanintroductiontocodingcomputerprogramsforreaderswithnopreviouscodingexperience.Althoughthisisabookforbeginners,itgoesbeyondthemerebasicssosometopicsmaybemoreeasilyunderstoodaftergainingsomecodingexperiencewiththesimplerlistedprograms.AlltheexamplesdemonstratecodingfeaturesusingthepopularPythonprogramminglanguageandthebook’sscreenshotsillustratetheactualresultsproducedbyexecutingthelistedcode.

Conventionsinthisbook

Inordertoclarifythecodelistedinthestepsgivenineachexample,IhaveadoptedthesamedefaultcolorizationconventionprovidedbyPython’scodeeditor.KeywordsofthePythonlanguageitselfarecoloredorange,built-infunctionnamesarepurple,coder-specifiedfunctionnamesareblue,textstringsaregreen,commentsarered,andallothercodeisblack,likethis:#Afunctiontodisplayagreeting

defgreet(reader):print(‘WelcometoCodingforBeginners’,reader)

Additionally,inordertoidentifyeachsourcecodefiledescribedinthesteps,aniconandfilenameappearsinthemarginalongsidethesteps,likethis:

program.py

Grabbingthesourcecode

ForconvenienceIhaveplacedsourcecodefilesfromtheexamplesfeaturedinthisbookintoasingleZIParchive.Youcanobtainthecompletearchivebyfollowingtheseeasysteps:

Browsetowww.ineasysteps.comthennavigatetoFreeResourcesandchoosetheDownloadssection

FindCodingforBeginnersineasystepsinthelist,thenclickonthehyperlinkentitledAllCodeExamplestodownloadthearchive

Now,extractthearchivecontentstoanyconvenientlocationonyourcomputer

1Gettingstarted

Welcometotheexciting,funworldofcomputercoding!Thischapterdescribeshowtocreateyourownprogramming

environmentanddemonstrateshowtocodeyourveryfirstprogram.

ProgrammingcodeSettingupExploringIDLEGettinghelpSavingprogramsStoringvaluesAddingcommentsNamingrulesSummary

ProgrammingcodeAcomputerismerelyamachinethatcanprocessasetofsimpleinstructionsveryquickly.Thesetofinstructionsitprocessesisknownasa“program”,andtheinstructionsareknownas“code”.

Peoplewhowritecomputerprogramsareknownas“programmers”or“coders”.Theirprogramshaveenabledcomputerstobecomeusefulinalmosteveryareaofmodernlife:

• Inthehand–computersarefoundincellphonedevicesfortaskssuchascommunicationviavoice,text,andsocialmedia

• Inthehome–computersarefoundinhouseholddevicessuchasTVsets,gamingconsoles,andwashingmachines

• Intheoffice–computersarefoundindesktopdevicesfortaskssuchaswordprocessing,payroll,andgraphicdesign

• Inthestore–computersarefoundinretaildevicessuchasautomatictellermachines(ATMs)andbarcodescanners

• Inthecar–computersarefoundincontroldevicesfortaskssuchasenginemanagement,anti-lockbrakingandsecurity

• Inthesky–computersarefoundinairplanesforpilotingandinairtrafficcontrolcentersforsafenavigation

Theseare,infact,justafewexamplesofhowcomputersaffectourlivestoday.Yet,computersarereallydumb!Theycanonlycountfromzerotoone,andcannotthinkforthemselves.

Acomputerisacollectionofelectroniccomponents–collectivelyknownas“hardware”.Tomakethecomputerfunctionitmustbegivenasetofprograminstructions–knownas“software”.

Itisimportantthateachcomputerprogramprovidesclearstep-by-stepinstructionsthatthecomputercanexecutewithouterrors.Thecodermustthereforebreakdownthetaskrequiredofthecomputerintosimpleunambiguoussteps.Forexample,aprogramtomoveamobilerobotfromindoorstooutdoorsmustincludeinstructionstohavetherobotlocateadoorwayandnavigatearoundanyobstacles.Sothecodermustalwaysconsiderwhatpossibleunexpecteddifficultiesaprogrammayencounter.

Programinstructionsmustbepresentedtothecomputerinalanguageitcanunderstand.Atthemostbasiclevelthecomputercanunderstand“machinecode”,whichmovesitemsaroundinitsmemorytoperformtasks.Thistypeofobscurelow-levelcodeisincredibly

tediousasitrequiresmanylinesofinstructiontoperformevenasimpletask.

Fortunately,overtheyears,many“high-level”programminglanguageshavebeendevelopedthatallowthecodertocomposeinstructionsinmorehuman-readableform.Thesemodernhigh-levelprogramsareautomaticallytranslatedintothemachinecodethatthecomputercanunderstandbya“compiler”orbyan“interpreter”.Inordertobecomeacoderyoumusttypicallylearnatleastoneofthesehigh-levelprogramminglanguages:

• C–apowerfulcompiledlanguagethatiscloselymappedtomachinecodeandusedtodevelopoperatingsystems

• C++–anenhancedcompiledlanguagedevelopingonCtoprovideclassesforObjectOrientedProgramming(OOP)

• C#–amoderncompiledlanguagedesignedbyMicrosoftforthe.NETframeworkandCommonLanguageInfrastructure

• Java–aportablecompiledlanguagethatisdesignedtorunonanyplatformregardlessofthehardwarearchitecture

• Python–adynamicinterpretedlanguagethatallowsbothfunctionalandObjectOrientedProgramming(OOP)

Programswritteninaninterpretedlanguagecanberunimmediatelybutthosewrittenincompiledlanguagesmustfirstbecompiledbeforetheycanberun.

Justashumanlanguageshavesimilarities,suchasverbsandnouns,theseprogramminglanguageshavecertainsimilaritiesastheyeachpossess“datastructures”,inwhichtostoreinformation,and“controlstructures”thatdeterminehowtheprogramproceeds.

TheexamplesinthisbookusethePythonlanguagetodemonstratehowtocodecomputerprogramsasithasasimplelanguagesyntax,requiresnocompilation,includesalargelibraryofstandardfunctions,andcanbeusedtocreatebothConsoleprogramsandwindowedGUI(GraphicalUserInterface)apps.

Pythonisatotalpackageof“batteriesincluded”.

SettingupBeforeyoucanbegincodingprogramsinthePythonlanguageyouneedtosetupaprogrammingenvironmentonyourcomputerbyinstallingthePythoninterpreterandthestandardlibraryoftestedcodemodulesthatcomesalongwithit.ThisisavailableonlineasafreedownloadfromthePythonSoftwareFoundation.

Launchawebbrowserandnavigatetopython.org/downloadsthenclicktheDownloadsbuttontograbthelatestversionforyoursystem–inthiscaseit’s“Python3.4.2”

Whenthedownloadcompletesruntheinstallerandchoosewhethertoinstallforallusersorjustyourself,thenclicktheNextbuttontoproceed

Now,acceptthesuggesteddefaultinstallationlocation,whichwillbeadirectoryonyourrootC:\drivenamed“Python”andversionnumber–inthisexampleit’sadirectoryatC:\Python34forPythonversion3.4.2

InstallersforMacOSXandOtherPlatformsarealsofreelyavailableatpython.org/downloads

Doacceptthesuggesteddestinationdirectory–suchasC:\Python34thatissuggestedhere.

ClicktheNextbuttontoproceedthenbesuretoselectthefeatureto“Addpython.exetoPath”

ClickonNexttobegincopyingfilesontoyourcomputerthenclicktheFinishbuttontocompletetheinstallation

UponcompletionthePythongroupisaddedtoyourStart/Appsmenu.MostimportantofthisgroupistheIDLEitemthatlaunchesthePythonintegrateddevelopmentenvironment.

AddingPythontothesystemPathmakesitavailablefromwithinanydirectory.Afterinstallation,youcanexactlyenterthecommandpython-VataCommandPrompttoseetheinterpreterrespondwithitsversionnumber.

YouwillusetheIDLElauncheroftensoright-clickonitsiconandchoose“Pintotaskbar”tomakeitreadilyavailablefromtheWindowsDesktop.

ExploringIDLETheinstalledPythonsoftwarepackageincludestheIntegratedDeveLopmentEnvironment(IDLE)inwhichyoucaneasilycodeandrunprograms,orsnippets,writteninthePythonlanguage.IDLEprovidestwodifferentwindowsforprogramdevelopment:

• ShellWindow

• EditWindow

WhenyoustartupIDLEitopensanewwindowcontainingamenubar,abannerdescribingtheversion,anda>>>prompt.ThisistheShellWindowinwhichyoucaninteractdirectlywiththePythoninterpreterbyenteringstatementsattheprompt.

Iftheinterpreterunderstandsyourentryitwillrespondwithanappropriatereply,otherwiseitwillreportanerror.

YoucanmaketheinterpreterprintoutastringoftextbyenteringaPythonprint()functionstatementthatenclosesyourstringwithinquotemarksinsidetheparenthesesattheinteractiveprompt.

Mostprogramminglanguagesrequiretextstringstobeenclosedinquotemarkstodifferentiatethemfromprogramcode.Byconvention,Pythoncodersusesinglequotes.

Youcanalsomaketheinterpreterprintouttheresultofasimplearithmeticsumbyenteringavalidsumstatementattheprompt.

Ifyourstatementisnotvalid,suchasasumthatattemptstodivideanumberbyzero,theinterpreterwillprintoutanerrormessagehelpfullydescribingthenatureoftheerror.

OpenanIDLEShellWindowthenpreciselyenterthisstatementattheinteractivepromptprint(‘HelloWorld!’)

Next,hittheReturnkeytoseetheinterpreter’sresponse

Now,enterthissumstatementattheinteractiveprompt

8+4

Spacesinstatementsareignored–so8+4canbeenteredwithoutspaces.

HitReturntoseetheinterpreterprinttheresulttotal

Enterthisinvalidstatementattheinteractiveprompt

8/0

HitReturntoseetheinterpreterprintanerrormessage

TheShellWindowismostlyusedtotestsnippetsofcode.

GettinghelpTheIDLEShellWindowprovidesagreatHelputilitywhereyoucanfindhelponanyPythontopicwhencodingPythonprograms.HelpcanbesoughtbyenteringaPythonhelp()statementattheinteractive>>>prompt.Awelcomemessageappearsandthepromptchangestohelp>todenoteyouarenowinHelpmode.

OpenanIDLEShellWindowthenpreciselyenterthisstatementattheinteractiveprompthelp()

Next,hittheReturnkeytoenterHelpmode

Now,enterthistopicnameattheHelputilitypromptkeywords

HitReturntolistallkeywordsofthePythonlanguage

TheHelputilitywelcomemessagealsocontainshandyhints–butareomittedhereforbrevity.

Keywordsarethevocabularyofaprogramminglanguage.NotethatPythonkeywordsarecase-sensitive–theseareallinlowercaseexceptFalse,None,andTrue.

Then,enterthiscommandattheHelputilitypromptquit

HitReturntoexitHelpandreturntoaninteractiveShellWindowprompt

Therearenoparenthesesrequiredafterthequitinstruction–hereitisaHelputilitycommand,notaPythonstatement.

Whenyoujustwanthelponasingletopicyoucansimplyenterthetopicnamewithinquotemarksinsidetheparenthesesofahelp()statementattheinteractiveprompt:

Preciselyenterthisstatementattheinteractiveprompt

help(‘keywords’)

HitReturntolistallkeywordsofthePythonlanguageandremainataninteractiveShellWindowprompt

Keywordshavespecialmeaninginaprogramminglanguage–theycannotbeusedtonameitemsinyourcode.

SavingprogramsTheIDLEShellWindow,describedonthepreviouspage,isagreatplacetotryoutsnippetsofcode,butcannotsaveyourcode.HappilyIDLEalsoprovidesanEditWindowwhereyoucancreatelongerpiecesofprogrammingcodethatcanbestoredina(.py)fileonyourcomputer.Thismeansyoucaneasilyre-runthecodewithoutre-typingalltheinstructionsattheShellWindow>>>promptandthisletsyouedityourcodetotrynewideas.Theproceduretocreate,save,andrunyourcodelookslikethis:

• OpenanEditWindowfromtheShellWindowbyselectingFile,NewFilefromtheShellWindowmenuitems–orbypressingtheCtrl+Nshortcutkeys

• TypecodeintotheEditWindowthensaveitbyselectingFile,SavefromtheEditWindowmenuitems–orbypressingtheCtrl+Sshortcutkeys

• RunsavedcodefromtheEditWindowbyselectingRun,RunModulefromtheEditWindowmenuitems–orbypressingtheF5shortcutkey

Theproceduredescribedherewillbeusedtodemonstratethecodeexamplesgiventhroughoutthisbook.

OutputfromyourprogramcodewillappearintheShellWindowastheprogramruns,orahelpfulerrormessagewillappearthereiftheinterpreterdiscoversanerrorinyourcode.

OpenanIDLEShellWindowthenselecttheFile,NewFilemenuitemtoopenanIDLEEditWindow

NoticetheFile,OpenandFile,RecentFilesmenuitemsthatcanbeusedtore-runprogramcodepreviouslysaved.

helloworld.py

Now,intheIDLEEditWindow,preciselyenterthiscode

print(‘HelloWorld!’)

Next,intheIDLEEditWindow,selecttheFile,Savemenuitems,toopentheSaveAsdialog,thensaveyourprogramcodeasafilenamedhelloworld.py

Finally,intheIDLEEditWindow,selecttheRun,RunModulemenuitems,torunyourprogramcodeandseetheoutputappearintheShellWindow

Yourprogramcodecanbesavedatanyconvenientlocationonyourcomputer–hereitissavedinadirectorycreatedatC:\MyCodethatwillbeusedforallexamplesinthisbook.

NoticethattheShellWindowrestartswheneveritrunsyourprogramcodeafresh.

StoringvaluesOneessentialfeatureofallcomputerprogramminglanguagesistheabilitytostoredatavaluesintheprogramcode.Thisabilityisprovidedbyasimpledatastructurecalleda“variable”.Avariableisacontainerinwhichanitemofdatacanbestored,muchlikeareal-lifeobjectcanbestoredinabox.

Whencreatingavariableyougiveitanameofyourchoice,subjecttothenamingconventionsoftheprogramminglanguage,thatactslikealabelonabox.Thedataitemstoredwithinthevariablecansubsequentlyberetrievedusingitsgivenname–justasyoucanfindareal-lifeobjectinaboxbyreadingitslabel.

DatatobestoredinavariableisassignedinaPythonprogramdeclarationstatementwiththe=assignmentoperator.Forexample,tostorethenumericvalueeightinavariablenamed“a”:a=8

Thestoredvaluecanthenberetrievedusingthevariable’sname,sothatthestatementprint(a)willoutputthestoredvalue8.Thatvariablecansubsequentlybeassignedadifferentvalue,soitsvaluecanvaryastheprogramproceeds–hencetheterm“variable”.

InPythonprogrammingavariablemustbeassignedaninitialvalue(“initialized”)inthestatementthatdeclaresitinaprogram–otherwisetheinterpreterwillreporta“notdefined”error.

Multiplevariablescanbeinitializedwithacommonvalueinasinglestatementusingasequenceof=assignments.Forexample,toinitializevariablesnamed“a”,“b”and“c”eachwithanumericvalueofeightlikethis:a=b=c=8

Someprogramminglanguages,suchasJava,demandyouspecifyinitsdeclarationwhattypeofdataavariablemaycontain.Thisreservesaspecificamountofmemoryspaceandisknownas“statictyping”.Pythonvariables,ontheotherhand,havenosuchlimitationandadjustthememoryallocationtosuitthevariousdatavaluesassignedtotheirvariables(“dynamictyping”).Thismeanstheycanstoreintegerwholenumbers,floating-pointnumbers,textstrings,orBooleanvaluesofTrueorFalseasrequired.

Programminglanguagesthatrequirevariabletypestobespecifiedare

alternativelyknownas“stronglytyped”,whereasthosethatdonotarealternativelyknownas“looselytyped”.

firstvar.py

OpenanIDLEEditWindowthenentercodetocreateavariablenamed“var”tostoreawholenumberintegervar=8

Next,addastatementtodisplaythestoredintegervalue

print(var)

Assignanewfloating-pointnumbertothevariablethenaddastatementtodisplaythestoredfloatvaluevar=3.142

print(var)

Now,assignatextstringtothevariablethenaddastatementtodisplaythestoredstringvaluevar=‘CodingforBeginnersineasysteps’print(var)

Finally,assignalogicaltruthvaluetothevariablethenaddastatementtodisplaythestoredBooleanvaluevar=True

print(var)

Savethefile(File,Save)thenruntheprogram(Run,RunModule)toseethestoredvaluesdisplayedinoutput

Textstringdatamustbeenclosedwithinquotemarkstodenotethestartandendofthatparticularstring.

AddingcommentsWhenyoubegintocodelongerprogramsitisusefultoaddcommentsatthestartofeachpieceofcodedescribingthepurposeofthatpiece.Thismakesthecodemoreeasilyunderstoodbyothers,andbyyourselfwhenrevisitingthecodeatalaterdate.InthePythonprogramminglanguageeverythingonasinglelineaftera#hashcharacterisignoredbytheinterpreter.Thismeansthatasingle-linecommentcanbeinsertedaftera#character.

comment.py

OpenanIDLEEditWindowthenentercommentedcodetoinitializeavariableanddisplayitsstatus#Initializeprogramstatus

running=Trueprint(‘Runstate:‘,running)

Savethefilethenruntheprogramtoseethecommentgetignoredandthestoredvaluedisplayedinoutput

Toreadilyidentifyaspectsofyourcode,IDLEautomaticallycolorizesyourcode,bothintheShellWindowandtheEditWindow,withthedefaultcolorslistedinthetablebelow:

CodelistedinthestepsthroughoutthisbookalsousethedefaultIDLEcolorsfor

consistency.

Namingrules

Keywords:

False None True

and as assert

break class continue

def del elif

else except finally

for from global

if import in

is lambda nonlocal

not or pass

raise return try

while with yield

Itisgoodprogrammingpracticetochoosemeaningfulnamesthatreflectthenatureofthevariable’scontent.

Variablecontainersthatyoucreateinyourcodetostoredatawithinaprogramcanbegivenanynameofyourchoosing–providingyoudonotuseanyoftheprogramminglanguagekeywords,suchasthePythonkeywordsinthetableabove,andthenameadherestothenamingruleslistedinthetablebelow:

Namingrule: Example:

CANNOTcontainanykeywords True

CANNOTcontainarithmeticoperators a+b*c

CANNOTcontainsymbols %$#@!

CANNOTcontainanyspaces nospaces

CANNOTstartwithanumber 2bad

CANcontainnumberselsewhere good1

CANcontainlettersofmixedcase UPdown

CANcontainunderscores is_ok

Variablenamesarecase-sensitiveinPython–sovariablesnamed“VAR”,“Var”,and“var”wouldbetreatedasthreeseparatevariables.

Summary• Acomputerprogramisasetofinstructions,writtenbyacoder,thatenablecomputers

tobecomeuseful

• Theelectroniccomponentsofacomputerareitshardware,whereasprograminstructionsareitssoftware

• Computersunderstandlow-levelmachinecode

• High-levelprogramminglanguagesinhuman-readableformgetautomaticallytranslatedintolow-levelmachinecode

• Programminglanguagespossessdatastructurestostoreinformationandcontrolstructurestodetermineprogress

• ThePythonprogramminglanguagehassimplesyntax,requiresnocompilation,andincludesalibraryoffunctions

• Python’sdevelopmentenvironmentiscalledIDLE

• IDLEprovidesaShellWindowcontaininganinteractivepromptfortestingandanEditWindowforcodingprograms

• TheIDLEHelputilityisaccessedbyenteringahelp()statementataShellWindowprompt

• AftertypingprogramcodeintoanIDLEEditWindowitmustfirstbesavedasafilebeforetheprogramcanberun

• OutputfromaprogramrunfromtheEditWindowappearsintheShellWindow,orahelpfulerrormessageappearsthere

• Avariabledatastructureisanamedcontainerthatallowsasingleitemofdatatobestoredforusebyaprogram

• Datastoredinavariablecanberetrievedusingthatvariable’snameandmaybereplacedbyassigninganewvalue

• VariablesinPythonprogrammingcanstoreanytypeofdata

• Commentlinescanusefullybeaddedtoprogramcodeafterbeginningthelinewitha#hashcharacter

• Variablenamesmustnotuseanyoftheprogramminglanguagekeywordsandmustadheretoitsnamingrules

2Savingdata

Thischapterdemonstrateshowtocreatecodetousevarioustypesofdatastoredinsideyourprograms.

StoringinputControllingoutputRecognizingtypesConvertingdataGuessinggameCorrectingerrorsSummary

StoringinputTheabilitytostoreandreplacecodedprogramdatainavariableisgreat,butthisabilitycanalsobeusedtostoredatainputbyauser–allowingyourprogramstobecomeinteractive.

InPythonprogrammingabuilt-ininput()functioncanbeusedtoacceptuserinputfromakeyboardandassignittoavariable.Optionally,thisfunctioncanspecifyastring,withinquotemarksinsideitsparentheses,thatwillbedisplayedtorequesttheinput.TheprogramwillwaituntiltheuserhitstheReturnkeybeforeassigningtheirinputtothevariableandproceedingonwards.

Storedvariablevaluescanbeoutputbythebuilt-inprint()functionbyspecifyingthevariablenamewithinthefunction’sparentheses.Multiplevaluesmayalsobespecifiedforoutputasacomma-separatedlistwithintheparentheses.

input.py

OpenanIDLEEditWindowthenenterthiscodetorequestuserinputwithwhichtoinitializeavariablename=input(‘Pleaseenteryourname:‘)

Next,addastatementtooutputbothastringandthevaluestoredwithinthevariableprint(‘Hello’,name)

Now,addstatementstooutputastringthenbothastringandthevaluestoredwithinthevariableoncemoreprint(‘WelcometoCodingforBeginners’)

print(‘Remembertohavefun’,name,‘!’)

Savethenruntheprogram,enteryournamewhenrequested,andhitReturntoseeyournameintheoutput

Thereisnoneedtoincludespacesinthecomma-separatedlists–theyareignoredbytheinterpreterbutareshownhereforclarity.

ControllingoutputAstheexampleonthefacingpagedemonstrates,avaluestoredinavariableremainsavailableforrepeateduseuntilitisreplacedbyanewvalueoruntiltheprogramends.

Therearealsotwopointsworthnotingwithregardtothisexample’soutput.Firstly,theprint()functionautomaticallyaddsaninvisible\nnewlinecharacterafteritsoutput–sothenextprint()functionoutputwillbydefaultappearonthenextlinebelow.Secondly,theprint()functionautomaticallyaddsaspacebetweeneachitemwhenacomma-separatedlistisspecifiedforoutput.

Youcanoverridetheautomaticnewlinebehaviorbyspecifyingthatthelineshouldendwithaspace,ratherthanthe\ndefault.Youcanalsoavoidtheautomaticspacingbehaviorby“concatenating”itemswitha+symbol,ratherthanacomma.

concat.py

OpenanIDLEEditWindowthenenterthiscodetorequestuserinputwithwhichtoinitializeavariablename=input(‘Pleaseenteryourname:‘)

Next,addastatementtooutputastringandthevaluestoredwithinthevariable–butwithoutafinalnewlineprint(‘Hello’+name,end=‘‘)

Now,addstatementstooutputastringthenastringconcatenatedtothevaluestoredwithinthevariableprint(‘WelcometoCodingforBeginners’)

print(‘Remembertohavefun’+name+‘!’)

Savethenruntheprogram,enteryournamewhenrequested,andhitReturntoseecontrolledoutput

Noticethatthestringshavebeeneditedtonicelyformattheoutput.

RecognizingtypesTherearefouressentialtypesofdatayoucanrepresentwhencodingacomputerprogramandthatcanbestoredinvariables.AlthoughvariablesinthePythonlanguagecanstoreanytypeofdataitisimportanttounderstandthedifferenttypesasyouwillsometimesneedtoconvertfromonetypetoanotherinyourcode:

Datatype: Description: Example:

str Astringofcharacters,whichcanincludeletters,numbers,spaces,andsymbols

‘Daytona500’

int Anintegerwholenumber,whichDOESNOThaveadecimalpointpart 1000

float Afloating-pointnumber,whichDOEShaveadecimalpointpart 98.6

bool ABooleanlogicaltruthvalue,whichiseitherTrueorFalse True

Integerintnumbersshouldnotincludeanypunctuation–socodeonethousandas1000ratherthan1,000.

Stringsaresimplyanycollectionofcharactersyoucanenterfromthecomputerkeyboard,groupedtogetherwithinquotemarks.Variablesrecognizeavaluebeingassignedtoitasbelongingtothestr“class”ifthatvalueisenclosedwithinquotemarks.

Stringstrvaluesmustalwaysbeenclosedwithquotemarks.Numbersareanynumericvalueyoucanenterfromthekeyboardbutaredefinedbytheinclusionoromissionofadecimalpoint.Variablesrecognizeanumericvaluebeingassignedtoitasbelongingtotheint“class”ifthatvaluehasnodecimalpointorasbelongingtothefloatclassifitdoesindeedhaveadecimalpoint.

Booleansarealogicaltruthvalue.VariablesrecognizeaBooleanvaluebeingassignedtoitasbelongingtothebool“class”bythekeywordsTrueandFalse,orifthatvalueevaluatestoTrueorFalse.

InPythonprogrammingyoucaneasilydiscoverthetypeofdatastoredwithinavariablebyspecifyingthatvariable’snamewithintheparenthesesofthebuilt-intype()function.

types.py

OpenanIDLEEditWindowandinitializeavariablebyassigningitastring,thendisplayitsvalueanddatatyperace=‘Daytona500’

print(race,‘is‘+type(race))

Next,initializeavariablebyassigningitawholenumberthendisplayitsvalueanddatatypekilo=1000print(kilo,‘is‘+type(kilo))

Now,initializeavariablebyassigningitadecimalnumber,thendisplayitsvalueanddatatypetemp=98.6

print(temp,‘is‘+type(temp))

Initializeavariablebyassigningitatruthkeywordthendisplayitsvalueanddatatypeflag=Trueprint(flag,‘is‘+type(flag))

Finally,replacethelastvariablevaluewithatruthresultofacomparison,thenagaindisplayitsvalueanddatatypeflag=4>8

print(flag,‘is‘+type(flag))

Savethenruntheprogramtodiscoverthetypesofdatastoredwithinthevariablesyouhavecreated

Thecomparisonhereexamineswhether4isgreaterthan8,whichisofcourseuntrue.Comparisonsaredemonstratedfullyinthenextchapter.

ConvertingdataOftenyouwillneedtoconvertdatainyourprogramstoperformsomekindofmanipulation–suchasarithmeticorconcatenation.Arithmeticcanonlybeperformedonnumericdatatypes,suchasintandfloatdata,whereasconcatenationcanonlybeperformedonstringdatatypes,suchasstrdata.

Pythonprovidesseveralbuilt-infunctionsthatallowyoutoeasilyconvertdatainyourprogramstoadifferentdatatype.Thesedonotconverttheoriginalspecifiedvalueitselfbutmerelyreturnaconvertedrepresentationofitsvalue.Inprogrammingtermsthisisknownasa“cast”operation:

Function: Description:

int(x) Convertsxtoanintegerwholenumber

float(x) Convertsxtoadecimalfloating-pointnumber

str(x) Convertsxtoastringrepresentation

Numericvaluesinyourcodeshouldnotbeenclosedwithinquotemarks–numberswithinquoteswillbeseenasstringvalues!

Numericvaluesassignedtovariablesmanuallyinyourcodeareautomaticallyappointedtheappropriatedatatypeofintorfloat.Valuesassignedtovariablesfromusersbythebuilt-ininput()functionare,however,alwaysautomaticallyappointedthestrstringdatatype–evenwhentheyaresimplynumericvalues!Thismeanstheymustbeconverted(cast)toanappropriatenumerictypeofintorfloatbeforeyoucanperformarithmeticwiththem.

Converselynumericvaluesassignedtovariablesmanuallyinyourcode,whichareautomaticallyappointedtheappropriatedatatypeofintorfloat,cannotbeconcatenatedintoastring.Thismeanstheymustbeconvertedtoastrdatatypeforinclusioninastring.

InPython,asinmanyotherprogramminglanguages,the+symbolhasmorethanonepurposeaccordingtoitscontext.Wherethe+symbolisusedbetweentwonumericvaluesitperformsanaddition(seenasan“additionoperator”)butwherethe+symbolisusedbetweentwostringvaluesitperformsaconcatenation(seenasa“concatenationoperator”).Variablesthatareassignedtheresultofeitherkindofoperationwillautomaticallybeappointedtheappropriatedatatypeoftheresult.

Arithmeticperformedonanintandfloatdatatypetogetherwillbeautomaticallycastintoafloatresult.Castvaluestothefloatdatatypetoallowfordecimalnumberinput.

cast.py

OpenanIDLEEditWindowandinitializetwovariablesbyassigningthemnumericuserinputnum1=input(‘Pleaseenterawholenumber:’)

num2=input(‘Nowenteranotherwholenumber:’)

Next,displaythedatatypeofeachvariabletoseethenumericvaluesare,infact,storedasstringsprint(‘Inputis:‘,type(num1),type(num2))

Now,usethe+operatortoattemptaddition,butseetheresultgetsconcatenatedasastrdatatypetotal=num1+num2print(‘Total:‘,total,type(total))

Again,usethe+operatortoattemptaddition,butcastthestoredvaluestoseetheresultasanintdatatypetotal=int(num1)+int(num2)

print(‘Total:‘,total,type(total))

Finally,castthestoredvaluesasafloatdatatypeandconcatenatethefloatresultvaluetotheoutputstrstringtotal=float(num1)+float(num2)print(‘Total:‘+str(total),type(total))

Savethenruntheprogramtoseethestoreddatatypesconvertedbycastingthemwiththebuilt-infunctions

Ifyouforgettoconverttothecorrectdatatypetheinterpreterwillreportanerror–tryaddinganinttoastrdatatypetoseetheerrormessage.

GuessinggameTheprevioussimpleexampleshaveillustratedhowvariablescanbeusedtostoretextstringvalues,numericintegerandfloating-pointdecimalvalues,andBooleantruthvaluesinyourprograms.Now,theycanbeusedtocreateaGuessingGameprogrambystoringarandomgeneratedintegerwhosevaluetheuserwillhavetoguess,aBooleantruthvaluethatwillendthegamewhentheuserguessescorrectly,andastringcontainingtheuser’sguess.

Thecodeinthisexampleincludessomefeaturesthatareonlydemonstratedlaterinthisbook,butasPythonisaneasilyhuman-readableprogramminglanguageyoushouldbeabletounderstandinprinciplehowthisprogramworks.

guess.py

OpenanIDLEEditWindowandbeginaprogrambyimportinga“random”libraryclassthatprovidesrandomnumbergeneratorfunctionsimportrandom

Now,initializethreevariables–ageneratedrandomnumberbetweenoneand20,aBooleanvaluethatwillremainTrueaslongasthegameisinprogress,andazerovaluethatwilllaterstoreanumberinputbytheusernum=random.randint(1,20)

flag=Trueguess=0

Next,displayamessageaskingtheusertomakeaguess

print(‘Guessmynumber1-20:‘,end=‘‘)

Then,preciselycopythiscodethatwillcomparethestoreduser’sguesstothestoredrandomnumberwhileflag==True:

guess=input()ifnotguess.isdigit():

print(‘Invalid!Enteronlydigits1-20‘)break

elifint(guess)<num:print(‘Toolow,tryagain:‘,end=‘‘)

elifint(guess)>num:print(‘Toohigh,tryagain:‘,end=‘‘)

else:print(‘Correct…Mynumberis‘+guess)

flag=False

Manyprogramlanguagesusecharacterssuchas{}togroupstatementstogetherbutPythonusesindentation–sotheindentationshownheremustbecorrectlypreserved.

Savethenruntheprogramandenterguessestoseeyourinputcomparedtoastoredrandomnumber

Noticethattheprogramrejectsuserinputofstrstringorfloatvalues.GuessingGameinPython–programanalysis

• Therandomlibraryclass’srandint()functionspecifiesanupperandlowerrandomrangewithinitsparentheses

• Theflagvariablespecifiesaninitialprogramconditionthatwillallowtheprogramtostartexaminingtheuser’sinput

• Thewhilekeywordspecifiesa“loop”controlstructurethatwillrepeatthestatementsitcontainsuntilatestedconditionfails

• Theifnotguess.isdigit()testspecifiesanactiontobreaktheloopiftheuserinputisnotanintegerwholenumber

• Theelifint(guess)<numtestspecifiesanalternativeactionifthecastuserinputislowerthanthestoredrandomnumber

• Theelifint(guess)>numtestspecifiesanalternativeactionifthecastuserinputishigherthanthestoredrandomnumber

• Theelsekeywordspecifiesafinalalternativeactiontochangetheprogramcondition,therebyendingtheprogram

Youneednotyetunderstandthisprogramindetail–eachaspectisexplainedbyexampleslaterinthisbook.

CorrectingerrorsWhencodingprogramstherearethreecommontypesoferrorthatcanoccur.ItisusefultorecognizethesedifferenterrortypesinPythonprogrammingsotheycanbecorrectedmoreeasily:

• SyntaxError–occurswhentheinterpreterencounterscodethatdoesnotconformtothePythonlanguagerules.Forexample,amissingquotemarkaroundastring.Theinterpreterhaltsandreportstheerrorwithoutexecutingtheprogram.

• RuntimeError–occursduringexecutionoftheprogram,atthetimewhentheprogramruns.Forexample,whenavariablenameislatermis-typedsothevariablecannotberecognized.Theinterpreterrunstheprogrambuthaltsattheerrorandreportsthenatureoftheerrorasan“Exception”.

• SemanticError–occurswhentheprogramperformsunexpectedly.Forexample,whenorderprecedencehasnotbeenspecifiedinanexpression.Theinterpreterrunstheprogramanddoesnotreportanerror.

Programmingerrorsareoftencalled“bugs”andtheprocessoftrackingthemdownisoftencalled“debugging”.

Correctingsyntaxandruntimeerrorsisfairlystraightforward,astheinterpreterreportswheretheerroroccurredorthenatureoftheerrortype,butsemanticerrorsrequirecodeexamination.

syntax.py

OpenanIDLEEditWindowthenaddastatementtooutputastringthatomitsaclosingquotemarkprint(‘CodingforBeginnersineasysteps)

Savethenruntheprogramtoseetheinterpreterhighlightthesyntaxerrorandindicateitsnature

TheredsyntaxerrorindicatorpointstothelinewheretheEndOfLine(EOL)erroroccurs.

Insertaquotemarkbeforetheclosingparenthesistoterminatethestringandsavethenruntheprogramagain–toseetheerrorhasbeencorrected

runtime.py

Next,beginanewprogrambyinitializingavariablethentrytooutputitsvaluewithanincorrectvariablename–toseetheinterpreterreportaruntimeerrortitle=‘CodingforBeginnersineasysteps’

print(titel)

Amendthevariablenametomatchthatinthevariabledeclarationandsavethenruntheprogramagain–toseetheerrorhasbeencorrected

semantic.py

Now,beginanewprogrambyinitializingavariablethentrytooutputanexpressionusingitsvaluewithoutexplicitprecedence–toseeapossiblyunexpectedresultof28

num=3

print(‘Result:‘,num*8+4)

Addparenthesestogrouptheexpressionas3*(8+4)thensavethefileandruntheprogramagain–toseetheexpectedresultof36,correctingthesemanticerror

DetailsofhowtohandleruntimeExceptionerrorsinyourscriptcodeareprovidedhere.

Summary• Variablescanbeusedtostoredatavaluesspecifiedinprogramcodeandtostoredata

valuesinputbyauser

• Avaluestoredinavariableremainsavailableforrepeateduseuntilitisreplacedbyanewvalueoruntiltheprogramends

• ThePythonbuilt-ininput()functioncanassigninputtoavariableandmayoptionallyspecifyastringtobedisplayed

• ThePythonbuilt-inprint()functioncanspecifymultiplevaluesforoutputasacomma-separatedlistinitsparentheses

• Bydefault,theprint()functionwillautomaticallyadda\nnewlineafteritsoutputunlessanendalternativeisspecified

• The+concatenationoperatorcanjointwostringstogether

• Datatypesareessentiallystrtextstrings,intintegernumbers,floatdecimalnumbers,orboolBooleantruthvalues

• BooleansarelogicaltruthvaluesthatarerepresentedinPythonbytheTrueandFalsekeywords

• ThePythonbuilt-intype()functioncanidentifytowhichclassdatatypeavalueinanyspecifiedvariablebelongs

• ValuesassignedtoavariablebythePythonbuilt-ininput()functionarealwaysappointedthestrstringdatatype

• DatatypescanbeconvertedformanipulationusingthePythonbuilt-inint(),float(),andstr()castingfunctions

• Arithmeticperformedonanintintegernumberandafloatdecimalnumberwillproduceafloatdatatyperesult

• Syntaxerrorsduetoincorrectcodearerecognizedbytheinterpreterbeforeexecutionoftheprogram

• Runtimeerrorsduetoexceptionsarerecognizedbytheinterpreterduringexecutionoftheprogram

• Semanticerrorsduetounexpectedperformancearenotrecognizedbytheinterpreter

3Performingoperations

Thischapterdemonstrateshowtouseoperatorsymbolsinyourcodestatements.

DoingarithmeticAssigningvaluesComparingvaluesFindingtruthTestingconditionSettingorderSummary

DoingarithmeticThearithmetical“operators”commonlyusedincodingcomputerprogramsusea+symbolforadditionanda-symbolforsubtraction,asyouwouldexpect.Typically,theyalsousean*asteriskformultiplication,ratherthananxsymbol,anda/forwardslashfordivision,ratherthana÷symbol.

ThearithmeticaloperatorsusedtocodePythonprogramsarelistedinthetablebelow,togetherwiththeoperationtheyperform:

Operator: Operation:

+ Addition

- Subtraction

* Multiplication

/ Division

% Remainder

// Floordivision

** Exponent

The+operatoraddstwonumberstogetherandthe-operatorsubtractsthesecondnumberfromthefirstnumber.

The*operatormultipliesthefirstnumberbythesecondnumberandthe/operatordividesthefirstnumberbythesecondnumber.

The%remainderoperatordividesthefirstnumberbythesecondnumberandreturnstheremainderoftheoperation.Thisisusefultodetermineifanumberhasanoddorevenvalue.

The//floordivisionoperatorperformsjustlikethe/divisionoperatorbuttruncatestheresultatthedecimalpoint–removinganyfloatingpointpartfromtheresultingnumber.

The**exponentoperatorreturnstheresultofthefirstnumberraisedtothepowerofthesecondnumber.

Valuesusedwithoperatorstoformexpressionsarecalled“operands”–intheexpression2+3thenumericalvalues2and3aretheoperands.

arithmetic.py

Startanewprogrambycreatingtwovariablescontainingwholenumbers(integers)

a=8

b=2

Next,displaytheresultofaddingthenumbers

print(‘Addition:\t’,a,‘+’,b,‘=’,a+b)

Now,displaytheresultofsubtractingthenumbers

print(‘Subtraction:\t’,a,‘-’,b,‘=’,a-b)

Then,displaytheresultofmultiplyingthenumbers

print(‘Multiplication:\t’,a,‘x’,b,‘=’,a*b)

Displaytheresultofdividingthenumbers,bothwithandwithoutthefloating-pointpartprint(‘Division:\t’,a,‘÷’,b,‘=’,a/b)print(‘FloorDivision:\t’,a,‘÷’,b,‘=’,a//b)

Next,displaytheremainderafterdividingthenumbers

print(‘Remainder:\t’,a,‘%’,b,‘=’,a%b)

Finally,displaytheresultofraisingthefirstnumbertothepowerofthesecondnumberprint(‘Exponent:\t‘,a,‘²=’,a**b,sep=‘’)

Savethenruntheprogramtoseetheresultofthearithmeticaloperations

Here,thespecial\tcharactersequenceaddsaninvisibletabcharactertoformattheoutput.

Youcanusethesepparametertoexplicitlyspecifytheseparationbetweenoutput–here,itspecifiesnospacesbyassigningtwounspacedsinglequotemarks.

AssigningvaluesTheassignmentoperatorcommonlyusedincodingcomputerprogramssimplyusesa=symboltoassignavaluetoavariable.Optionally,thismaybecombinedwithanarithmeticaloperatortoperformarithmeticandassignmentinonesingleoperation.

TheassignmentoperatorsusedtocodePythonprogramsarelistedinthetablebelowtogetherwiththeoperationtheyperform:

Operator: Example: Equivalent:

= a=b a=b

+= a+=b a=(a+b)

-= a-=b a=(a-b)

*= a*=b a=(a*b)

/= a/=b a=(a/b)

%= a%=b a=(a%b)

//= a//=b a=(a//b)

**= a**=b a=(a**b)

Intheexampleabove,variableaisassignedthevaluecontainedinvariableb–sothatbecomesthenewvalueintheavariable.

The+=operatorisusefultoaddavalueontoanexistingvaluestoredintheavariable–makingitacombinedtotalvalue.

Intheexampleabovethe+=operatorfirstaddsthevaluecontainedinvariableatothevaluecontainedinvariableb.Itthenassignstheresulttobecomethenewvaluestoredinvariablea.

Alltheseotheroperatorsworkinthesamewaybymakingthearithmeticaloperationbetweenthetwovaluesfirst,thenassigningtheresultofthatoperationtothefirstvariable–tobecomeitsnewstoredvalue.

Withthe%=operatorthefirstoperandaisdividedbythesecondoperandbthentheremainderisassignedtotheavariable.

Itisimportanttoregardthe=operatortomean“assign”ratherthan“equals”toavoidconfusionwiththe==equalityoperator.

assign.py

Startanewprogrambycreatingtwovariablescontainingintegernumbersanddisplaysbothassignedvaluesa=8

b=4print(‘AssignValues:\t’,’a=’,a,‘\tb=’,b)

Next,addandassignanewvaluetothefirstvariableanddisplayitsstoredvalue

a+=b

print(‘Add&Assign:\t’,’a=’,a,‘\t(8+=4)’)

Now,subtractandassignanewvaluetothefirstvariableanddisplayitsstoredvalue,thenmultiplyandassignavaluetothefirstvariableanddisplayitsstoredvaluea-=bprint(‘Subtract&Assign:\t’,’a=’,a,‘\t(12-4)’)

a*=bprint(‘Multiply&Assign:\t’,’a=’,a,‘\t(8x4)’)

Finally,divideandassignanewvaluetothefirstvariableanddisplayitsstoredvalue,thenremainderandassignavaluetothefirstvariableanddisplayitsstoredvaluea/=b

print(‘Divide&Assign:\t’,’a=’,a,‘\t(32÷4)’)a%=b

print(‘Remainder&Assign:\t’,’a=’,a,‘\t(8%4)’)

Savethenruntheprogramtoseetheresultoftheassignmentoperations

Unlikethe=assignoperator,the==equalityoperatorcomparesoperandsandisdescribedhere.

ComparingvaluesTheoperatorsthatarecommonlyusedinPythonprogrammingtocomparetwooperandvaluesarelistedinthetablebelow:

Operator: Comparativetest:

== Equality

!= Inequality

> Greaterthan

< Lessthan

>= Greaterthanorequalto

<= Lessthanorequalto

The==equalityoperatorcomparestwooperandsandwillreturnTrueifbothareequalinvalue,otherwiseitwillreturnaFalsevalue.Ifbotharethesamenumbertheyareequal,orifbotharecharacterstheirASCIIcodevaluesarecomparednumericallytoachievethecomparisonresult.

Conversely,the!=inequalityoperatorreturnsTrueiftwooperandsarenotequal,usingthesamerulesasthe==equalityoperator,otherwiseitreturnsFalse.Equalityandinequalityoperatorsareusefulintestingthestateoftwovariablestoperformconditionalbranchinginaprogramaccordingtotheresult.

The>“greaterthan”operatorcomparestwooperandsandwillreturnTrueifthefirstisgreaterinvaluethanthesecond,oritwillreturnFalseifitisequalorlessinvalue.The<“lessthan”operatormakesthesamecomparisonbutreturnsTrueifthefirstoperandislessinvaluethanthesecond,otherwiseitreturnsFalse.A>“greaterthan”or<“lessthan”operatorisoftenusedtotestthevalueofaniterationcounterinaloop.

Addingthe=operatoraftera>“greaterthan”or<“lessthan”operatormakesitalsoreturnTrueifthetwooperandsareexactlyequalinvalue.

A-ZuppercasecharactershaveASCIIcodevalues65-90anda-zlowercasecharactershaveASCIIcodevalues97-122.

comparison.py

Startanewprogrambyinitializingfivevariableswithvaluesforcomparison

nil=0

num=0top=1

cap=‘A’low=‘a’

Next,addstatementstodisplaytheresultsofnumericandcharacterequalitycomparisonsprint(‘Equality:\t’,nil,‘==’,num,nil==num)

print(‘Equality:\t’,cap,‘==’,low,cap==low)

Now,addastatementtodisplaytheresultofaninequalitycomparison

print(‘Inequality:\t’,nil,‘!=’,top,nil!=top)

Then,addstatementstodisplaytheresultsofgreaterandlessercomparisons

print(‘Greater:\t’,nil,‘>’,top,nil>top)print(‘Lesser:\t’,nil,‘<’,top,nil<top)

Finally,addstatementstodisplaytheresultsofgreaterorequalandlesserorequalcomparisonsprint(‘MoreOrEqual:\t’,nil,‘>=’,num,nil>=num)

print(‘LessorEqual:\t’,top,‘<=’,num,top<=num)

Savethenruntheprogram–toseetheresultofcomparisonoperations

The\tescapesequenceshownhereaddsaninvisibletabcharactertoformattheoutput.

TheASCIIcodevalueforuppercase“A”is65butforlowercase“a”it’s97–sotheircomparisonherereturnsFalse.

FindingtruthThelogicaloperatorsmostcommonlyusedinPythonprogrammingarelistedinthetablebelow:

Operator: Operation:

and LogicalAND

or LogicalOR

not LogicalNOT

ThelogicaloperatorsareusedwithoperandsthathaveBooleanvaluesofTrueorFalse,orarevaluesthatconverttoTrueorFalse.

The(logicalAND)andoperatorwillevaluatetwooperandsandreturnTrueonlyifbothoperandsthemselvesareTrue.OtherwisetheandoperatorwillreturnFalse.Thisisusefulinprogrammingtoperform“conditionalbranching”wherethedirectionofaprogramisdeterminedbytestingtwoconditions–ifbothconditionsaresatisfied,theprogramwillgoinacertaindirection,otherwiseitwilltakeadifferentdirection.

UnliketheandoperatorthatneedsbothoperandstobeTrue,the(logicalOR)oroperatorwillevaluateitstwooperandsandreturnTrueifeitheroneoftheoperandsitselfreturnsTrue.IfneitheroperandreturnsTrue,thentheoroperatorwillreturnFalse.Thisisusefulinprogrammingtoperformacertainactionifeitheroneoftwotestconditionshasbeenmet.

The(logicalNOT)notoperatorisaunaryoperatorthatisusedbeforeasingleoperand.Itreturnstheinversevalueofthegivenoperand,soifthevariableahadavalueofTruethennotawouldhaveavalueofFalse.Thenotoperatorisusefulinprogrammingtotogglethevalueofavariableinsuccessiveloopiterationswithastatementlikea=nota.ThisensuresthatoneachiterationofthelooptheBooleanvalueisreversed,likeflickingalightswitchonandoff.

Theterm“Boolean”referstoasystemoflogicalthoughtdevelopedbytheEnglishmathematicianGeorgeBoole(1815-1864).

logic.py

StartanewprogrambyinitializingtwovariableswithBooleanvaluesforlogicalevaluationa=True

b=False

AddstatementstodisplaytheresultsofANDevaluations

print(‘ANDLogic:’)print(‘aanda=’,aanda)

print(‘aandb=’,aandb)print(‘bandb=’,bandb)

AddstatementstodisplaytheresultsofORevaluations

print(‘\nORLogic:’)

print(‘aora=’,aora)print(‘aorb=’,aorb)

print(‘borb=’,borb)

AddstatementstodisplaytheresultsofNOTevaluations

print(‘\nNOTLogic:’)print(‘a=’,a,‘\tnota=’,nota)

print(‘b=’,b,‘\tnotb=’,notb)

Savethenrunthisprogram–toseetheresultoflogicoperations

InPythonprogramming,BooleanvaluescanalsoberepresentednumericallywhereTrueis1andFalseis0(zero).

NotethattheexpressionFalseandFalsereturnsFalse,notTrue–perhapsdemonstratingthemaxim“twowrongsdon’tmakearight”.

TestingconditionManyprogramminglanguages,suchasC++orJava,havea?:“ternary”operatorthatevaluatesanexpressionforaTrueorFalseconditionthenreturnsoneoftwospecifiedvaluesdependingontheresultoftheevaluation.A?:ternaryoperatorhasthissyntax:(test-expression)?if-true-return-this:if-false-return-this

Unlikeotherprogramminglanguages,Pythondoesnothavea?:

ternaryoperatorbuthasinsteada“conditionalexpression”thatworksinasimilarwayusingifandelsekeywordswiththissyntax:if-true-return-thisif(test-expression)elseif-false-return-this

Althoughtheconditionalexpressionsyntaxcaninitiallyappearconfusingitiswellworthbecomingfamiliarwiththisexpressionasitcanexecutepowerfulprogrambranchingwithminimalcode.Forexample,tobranchwhenavariableisnotavalueofone:if-true-do-thisif(var!=1)elseif-false-do-this

TheconditionalexpressioncanbeusedinPythonprogrammingtoassignthemaximumorminimumvalueoftwovariablestoathirdvariable.Forexample,toassignaminimumlikethis:c=aif(a<b)elseb

TheexpressioninparenthesesreturnsTruewhenthevalueofvariableaislessthanthatofvariableb–sointhiscasethelesservalueofvariableagetsassignedtovariablec.

Similarly,replacingthe<lessthanoperatorinthetestexpressionwiththe>greaterthanoperatorwouldassignthegreatervalueofvariablebtovariablec.

Anothercommonuseoftheconditionalexpressionincorporatesthe%remainderoperatorinthetestexpressiontodetermineifthevalueofavariableisanoddnumberoranevennumber:if-true(odd)-do-thisif(var%2!=0)elseif-false(even)-do-this

Wheretheresultofdividingthevariablevaluebytwodoesleavearemainder,thenumberisodd–wherethereisnoremainderthenumberiseven.Thetestexpression(var%2==1)wouldhavethesameeffectbutitispreferabletotestforinequality–it’seasiertospotwhensomethingisdifferentthanwhenit’sidentical.

Ingeneralprogrammingtermsan“expression”alwaysreturnsavalue,whereasa“statement”neednot–butastatementmayincludeoneormoreexpressions.

Theconditionalexpressionhas,ineffect,threeoperands–thetestexpressionandtwopossiblereturnvalues.

condition.py

Startanewprogrambyinitializingtwovariableswithintegervaluesforconditionalevaluationa=1

b=2

Next,addstatementstodisplaytheresultsofconditionalevaluation–describingthefirstvariable’svalueprint(‘\nVariableaIs:’,‘One’if(a==1)else‘NotOne’)print(‘VariableaIs:’,‘Even’if(a%2==0)else‘Odd’)

Now,addstatementstodisplaytheresultsofconditionalevaluation–describingthesecondvariable’svalueprint(‘\nVariablebIs:’,‘One’if(b==1)else‘NotOne’)

print(‘VariablebIs:’,‘Even’if(b%2==0)else‘Odd’)

Then,addastatementtoassigntheresultofaconditionalevaluationtoanewvariabletop=aif(a>b)elseb

Finally,addastatementtodisplaytheassignedresult–identifyingthegreaterofthetwovariablevaluesprint(‘\nGreaterValueIs:’,top)

Savethenrunthisprogram–toseetheresultofconditionalexpressionoperations

YoumayfindthatsomePythonprogrammersdislikeconditionalexpressionsastheyconsideritssyntaxcontradictstheprincipleofeasyreadability.

SettingorderOperatorprecedencedeterminestheorderinwhichexpressionsgetevaluated.Forexample,intheexpression3*8+4thedefaultorderofprecedencedeterminesthatmultiplicationwiththe*multiplyoperatoriscompletedfirst,sotheresultis28(24+4).

ThetablebelowlistsPython’soperatorprecedenceindescendingorder–thoseonthetoprowhavehighestprecedence,thoseonlowerrowshavesuccessivelylowerprecedence.TheprecedenceofoperatorsonthesamerowischainedLeft-To-Right.

Operator: Description:

** Exponent

+ Positive

- Negative

~ BitwiseNOT

* Multiplication

/ Division

// Floordivision

% Remainder

+ Addition

- Subtraction

| BitwiseOR

^ BitwiseXOR

& BitwiseAND

>> Bitwiserightshift

<< Bitwiseleftshift

>,>=,<,<=,==,!= Comparison

=,%=,/=,//=,-=,+=,*=,**= Assignment

is,isnot Identity

in,notin Membership

not BooleanNOT

and BooleanAND

or BooleanOR

The*multiplyoperatorisonahigherrowthanthe+additionoperator–sointheexpression3*8+4multiplicationiscompletedfirst,beforetheaddition.

Bitwiseoperatorsareusedforlow-levelmanipulationsoareomittedfromthisbook.Identityandmembershipoperatorsareintroducedlaterbutareincludedhereforcompleteness.

order.py

Startanewprogrambyinitializingthreevariableswithintegervaluesforprecedencecomparisona=2

b=4c=8

Next,addstatementstodisplaytheresultsofdefaultprecedenceandforcingadditionbeforemultiplicationprint(‘\nDefaultOrder:\t’,a,‘*’,c,’+’,b,‘=’,a*c+b)

print(‘ForcedOrder:\t’,a,‘*(‘,c,’+’,b,‘)=’,a*(c+b))

Now,addstatementstodisplaytheresultsofdefaultprecedenceandforcingsubtractionbeforedivisionprint(‘\nDefaultOrder:\t’,c,‘//’,b,‘-’,a,‘=’,c//b-a)

print(‘ForcedOrder:\t’,c,‘//(‘,b,’-’,a,‘)=’,c//(b-a))

Finally,addstatementstodisplaytheresultsofdefaultprecedenceandforcingadditionbeforeremainderoperationandbeforeexponentoperationprint(‘\nDefaultOrder:\t’,c,‘%’,a,‘+’,b,‘=’,c%a+b)print(‘ForcedOrder:\t’,c,‘%(‘,a,‘+’,b,‘)=’,c%(a+b))

print(‘\nDefaultOrder:\t’,c,‘**’,a,‘+’,b,‘=’,c**a+b)print(‘ForcedOrder:\t’,c,‘**(‘,a,‘+’,b,‘)=’,c**(a+b))

Savethenrunthisprogram–toseetheresultsofdefaultandexplicitprecedence

The//floordivisionoperatortruncatesfloatingpointvaluesatthedecimalpoint–butthe/divisionoperatorretainsthem.

Donotrelyupondefaultprecedence–alwaysuseparenthesestoclarifyyourexpressions.

Summary• Arithmeticaloperatorscanformexpressionswithtwooperandsforaddition+,

subtraction-,multiplication*,division/,floordivision//,remainder%,orexponent**

• The%remainderoperatorisusefultodeterminewhetheranumberisanoddorevenvalue

• Floordivisionwiththe//operatorremovesanyfloating-pointpartfromtheresult

• Theassignment=operatorcanbecombinedwithanarithmeticaloperatortoperformanarithmeticalcalculationthenassignitsresult

• The+=operatorisusefultoaddavalueontoanexistingvaluestoredinavariabletomakeacombinedtotalvalue

• Comparisonoperatorscanformexpressionscomparingtwooperandsforequality==,inequality!=,greater>,lesser<,greaterorequal>=,andlesserorequal<=values

• LogicalandandoroperatorsformexpressionsevaluatingtwooperandstoreturnaBooleanvalueofTrueorFalse

• ThelogicalnotoperatorreturnstheinverseBooleanvalueofasingleoperand-turningTruetoFalse,andviceversa

• ThenotoperatorisusefultotoggleavaluebetweenTrueandFalseonsuccessiveiterationsofaloop

• Aconditionalif-elseexpressionevaluatesagivenexpressionforaBooleanTrueorFalsevaluethenreturnsoneoftwooperandsdependingonitsresult

• Expressionscontainingmultipleoperatorswillexecutetheiroperationsinaccordancewiththedefaultprecedencerulesunlessexplicitlydeterminedbytheadditionofparentheses()

• Typically,addition+andsubtraction-willbeperformedbeforemultiplication*anddivision/unlessexplicitlyspecified

4Makinglists

Thischapterdemonstrateshowtocreatecodetostoredatainlistsandhowtoretrievedatafromlists.

WritinglistsChanginglistsFixinglistsSettinglistsNamingelementsSummary

WritinglistsSomeprogramminglanguagesallowvariablestobedeclaredwithnoinitialvalue,butinPythonprogrammingavariablemustbeassignedaninitialvalue(initialized)inthestatementthatdeclaresit–otherwisetheinterpreterwillreporta“notdefined”error.

Multiplevariablescanbeinitializedwithacommonvalueinasinglestatementusingasequenceof=assignments.Forexample,tosimultaneouslyassignacommonvaluetothreevariables:a=b=c=10

Alternatively,multiplevariablescanbeinitializedwithdifferingvaluesinasinglestatementusingcommaseparators.Forexample,tosimultaneouslyassigndifferentvaluestothreevariables:a,b,c=1,2,3

Unlikearegularvariable,whichcanonlystoreasingleitemofdata,anarrayvariablecanstoremultipleitemsofdata.InPythontheseareknownas“list”variables.Thedataisstoredsequentiallyinlist“elements”thatareindexnumberedstartingatzero.Sothefirstvalueisstoredinelementzero,thesecondvalueisstoredinelementone,andsoon.Anarraylistiscreatedmuchlikeanyothervariablebutisinitializedbyassigningvaluesasacomma-separatedlistbetweensquarebrackets.Forexample,creatingalistnamed“nums”likethis:nums=[0,1,2,3,4,5]

Asinglelistelementcanbereferencedusingthelistnamefollowedbysquarebracketscontainingthatelement’sindexnumber.Thismeansthatnums[1]referencesthesecondelementintheexampleabove–notthefirstelement.Aselementnumberingstartsatzeroitsfirstelementisreferencedwithnums[0].

Listscanhavemorethanoneindex–torepresentmultipledimensions,ratherthanthesingledimensionofaregularlist.Multi-dimensionallistsofthreeindicesandmoreareuncommonbuttwo-dimensionallistsareusefultostoregrid-basedinformationsuchasX,Ycoordinates.

Alistofstringvaluescanevenbeconsideredtobeamulti-dimensionallistaseachstringisitselfalistofcharacters.Soeachcharactercanbereferencedbyitsindexnumberwithin

itsparticularstring.

list.py

Startanewprogrambyinitializingalistofthreeelementscontainingstringvalues

quarter=[‘January’,‘February’,‘March’]

Next,addstatementstoindividuallydisplaythevaluecontainedineachlistelementprint(‘FirstMonth:’,quarter[0])

print(‘SecondMonth:’,quarter[1])print(‘ThirdMonth:’,quarter[2])

Addastatementtocreateamulti-dimensionallistoftwoelements,whichthemselvesareliststhateachhavethreeelementscontainingintegervaluescoords=[[1,2,3],[4,5,6]]

Now,addstatementstodisplaythevaluescontainedintwospecificinnerlistelementsprint(‘\nTopLeft0,0:’,coords[0][0])

print(‘BottomRight1,2:’,coords[1][2])

Finally,addastatementtodisplayjustonecharacterofastringvalue

print(‘\nSecondMonthFirstLetter:’,quarter[1][0])

Savethenruntheprogram–toseethelistelementvaluesgetdisplayed

Stringindicesmayalsobenegativenumbers–tostartcountingfromtherightwhere-1referencesthelastletter.

Loopstructures,whichareintroducedlaterinthischapter,areoftenusedtoiteratethroughlistelements.

ChanginglistsListvariables,whichcancontainmultipleitemsofdata,arewidelyusedinPythonprogrammingandhavevariousfunction“methods”thatcanbe“dot-suffixed”tothelistnameformanipulation:

ListMethod: Description:

list.append(x) Addsitemxtotheendofthelist

list.extend(L) AddsallitemsinlistLtotheendofthelist

list.insert(i,x) Insertsitemxatindexpositioni

list.remove(x) Removesfirstitemxfromthelist

list.pop(i) Removesitematindexpositioniandreturnsit

list.index(x) Returnstheindexpositioninthelistoffirstitemx

list.count(x) Returnsthenumberoftimesxappearsinthelist

list.sort() Sortalllistitems,inplace

list.reverse() Reversealllistitems,inplace

Forliststhatcontainbothnumericalandstringvaluesthesort()methodreturnsthelistelementssortedfirstnumericallythenalphabetically–forexampleas1,2,3,A,B,C.

Pythonalsohasausefullen(L)functionthatreturnsthelengthofthelistLasthetotalnumberofelementsitcontains.Liketheindex()andcount()methodsthereturnedvalueisnumericsocannotbedirectlyconcatenatedtoatextstringforoutput.

Stringrepresentationofnumericvaluescan,however,beproducedbyPython’sstr(n)functionforconcatenationtootherstrings,whichreturnsastringversionofthenumericnvalue.Similarly,astringrepresentationofanentirelistcanbereturnedbythestr(L)functionforconcatenationtootherstrings.Inbothcasesrememberthattheoriginalversionremainsunchanged,asthereturnedversionsaremerelycopiesoftheoriginal

version.

IndividuallistelementscanbedeletedbyspecifyingthelistnameandindexnumberafterthePythondelkeyword.Thiscanremoveoneelementataspecifiediindexposition,ora“slice”ofelementsusingslicenotationi1:i2tospecifytheindexnumberofthefirstandlastelement.Inthiscase,i1istheindexnumberofthefirstelementtoberemovedandallelementsupto,butnotincluding,theelementatthei2indexnumberwillberemoved.

Pythonalsohasanint(s)functionthatreturnsanumericversionofthestringsvalue.

pop.py

Startanewprogrambyinitializingtwolistsofthreeelementseachcontainingstringvaluesbasket=[‘Apple’,‘Bun’,‘Cola’]

crate=[‘Egg’,‘Fig’,‘Grape’]

Next,addstatementstodisplaythecontentsofthefirstlist’selementsanditslengthprint(‘BasketList:’,basket)print(‘BasketElements:’,len(basket))

Now,addstatementstoaddanelementanddisplayalllistelements,thenremovethefinalelementanddisplayalllistelementsoncemorebasket.append(‘Damson’)

print(‘Appended:’,basket)print(‘LastItemRemoved:’,basket.pop())

print(‘BasketList:’,basket)

Finally,addstatementstoaddallelementsofthesecondlisttothefirstlistanddisplayallthefirstlistelements,thenremoveelementsanddisplaythefirstlistagainbasket.extend(crate)print(‘Extended:’,basket)

delbasket[1]print(‘ItemRemoved:‘,basket)

delbasket[1:3]

print(‘SliceRemoved:’,basket)

Savethenrunthisprogram–toseelistsgetmanipulated

Thelastindexnumberintheslicedenotesatwhatpointtostopremovingelements–buttheelementatthatpositiondoesnotgetremoved.

FixinglistsThevaluesinaregularlistcanbechangedastheprogramproceeds.Eachelementmaybeassignedareplacementvalueusingthe=assignmentoperatortospecifythelistname,elementindexnumber,andthereplacementvalue.Inprogrammingtermsthevaluesstoredinaregularlistare“mutable”–theycanbechanged.Elementscanalsobedynamicallyaddedtoaregularlist,anddynamicallyremovedfromaregularlist,astheprogramproceeds.Thismeansthataregularlistisidealifyourprogramwillmakechangestoelementvalues.

Wherealistwillonlycontainconstantvalues,thatwillneverchangewhentheprogramruns,afixedlistcanbetterbecreated.Inprogrammingterms,thevaluesstoredinafixedlistare“immutable”–theycannotbechanged.Elementscannotbedynamicallyaddedtoafixedlist,ordynamicallyremovedfromafixedlist,astheprogramproceeds.Thismeansthatafixedlistisidealifyourprogramwillnevermakechangestoelementvalues.

Likeindexnumbering,withliststheitemsinatuplesequencearenumberedfromzero.

ArestrictiveimmutablePythonlistisknownasa“tuple”andiscreatedbyassigningvaluesasacomma-separatedlistbetweenparenthesesinaprocessknownas“tuplepacking”:colors_tuple=(‘Red’,‘Green’,‘Red’,‘Blue’,‘Red’)

Anindividualtupleelementcanbereferencedusingthetuplenamefollowedbysquarebracketscontainingthatelement’sindexnumber.Usefully,allvaluesstoredinsideatuplecanbeassignedtoindividualvariablesinaprocessknownas“sequenceunpacking”:a,b,c,d,e=colors_tuple

Regularlistmethods,suchassort()andreverse(),cannotbeusedontuplesbutthebuilt-inPythontype()functioncanbeusedtorevealthedataclasstypeandthebuilt-inlen()functioncanbeusedtoreturnthelengthofthetuple.

Theremustbethesamenumberofvariablesasitemstounpackatuple.

Typically,atupleisusedtostorevaluesthatareacollectionofconstantunchangingvaluessuchasday-of-the-weeknames,month-of-the-yearnames,orpersonaldetailsoffirstname,lastname,date-of-birth,address,phonenumber,etc.

tuple.py

Startanewprogrambyinitializingatuplethendisplayitsclasstype

days=(‘Mon’,‘Tue’,‘Wed’,‘Thu’,‘Fri’,‘Sat’,‘Sun’)

print(‘days:’,type(days))

Next,displaytheentirecontentsofthetuple,itslength,andthevaluestoredinitsfirstelementprint(‘Daysoftheweek:’,days)print(‘No.ofdaysinweek:’,len(days))

print(‘Startdayofweek:’,days[0])

Now,initializeanothertuplecontainingsomepersonaldetailsofauser

user=(‘John’,‘Doe’,‘Paris’,‘555-1234’)

Then,displaytheuser’sfullname

print(‘Name:’,user[0],user[1])

Finally,displaytheuser’sphonenumber

print(‘Phone:’,user[3])

Savethenrunthisprogram–toseethetuplevalues

Atuplemaycontainitemsthatarenotuniquetoitsotherelements.

SettinglistsThevaluesinaregularlistorafixedlisttuplecanberepeatedinitselements,butalistofuniquevaluescanbecreatedwhereduplicationisnotallowed.ArestrictivePythonlistofuniquevaluesisknownasa“set”andiscreatedbyassigningvaluesasacomma-separatedlistbetweencurlybrackets(braces)likethis:phonetic_set={‘Alpha’,‘Bravo’,‘Charlie’}

Asetmaynotcontainitemsthatarenotuniquetoitsotherelements.

Unlikeregularlistsortuples,individualelementsinasetcannotbereferencedusingthesetnamefollowedbysquarebracketscontaininganindexnumber.Instead,setshavepowerfulmethodsthatcanbedot-suffixedtothesetnameformanipulationandcomparisonofvalues:

SetMethod: Description:

set.add(x) Addsitemxtotheset

set.update(x,y,z) Addsmultipleitemstotheset

set.copy() Returnsacopyoftheset

set.pop() Removesonerandomitemfromtheset

set.discard(i) Removesitematpositionifromtheset

set1.intersection(set2) Returnsitemsthatappearinbothsets

set1.difference(set2) Returnsitemsinset1butnotinset2

Thebuilt-inPythontype()functioncanbeusedtorevealthedataclasstypeandthebuilt-inlen()functioncanbeusedtoreturnthelengthoftheset.Additionally,thePythonbuilt-inmembershipoperatorincanbeusedtofindvaluesinaset.

MoresetmethodscanbefoundinthePythondocumentationonlineatdocs.python.orgTypically,asetisusedtostoreuniquevaluesthatareacollectionofchangeablevalues,whichcaneasilybesearchedandcomparedusingthepowerfulsetmethods.Althoughyoucannotaccesssetelementvaluesbyindex,asetcanbeconvertedtoaregularlistusingthePythonbuilt-inlist()functiontoallowelementaccess.

set.py

Startanewprogrambyinitializingasetwithuniquenamevaluesthendisplayitsclasstypeparty_goers={‘Andrew’,‘Barbara’,‘Carole’,‘David’}

print(‘party_goers:’,type(party_goers))

Next,addstatementstosearchthesetelementsfortwospecifiedvalues

print(‘DidDavidgototheparty?’,‘David’inparty_goers)print(‘DidKellygototheparty?’,‘Kelly’inparty_goers)

Now,initializeanothersetwithuniquenamevalues

students={‘Andrew’,‘Kelly’,‘Lynn’,‘David’}

Then,createafurthersetcontainingonlycommonvaluesthatappearinbothprevioussetscommons=party_goers.intersection(students)

Initializearegularlistofthecommonvalues–sotheelementsvaluescanbeindividuallyaccessedparty_students=list(commons)

Finally,displayallcommonvaluesandthevaluestoredinthefirstregularlistelementprint(‘Studentsattheparty:’,party_students)

print(‘Firststudentattheparty:’,party_students[0])

Savethenrunthisprogram–toseethesetvalues

Noticethatthelist()functionmaynotplaceelementvaluesinthesameorderastheyappearintheset.

NamingelementsInPythonprogramminga“dictionary”isadatacontainerthatcanstoremultipleitemsofdataasalistofkey:valuepairs.Unlikeregularlistcontainervalues,whicharereferencedbytheirindexnumber,valuesstoredindictionariesarereferencedbytheirassociatedkey.Thekeymustbeuniquewithinthatdictionaryandistypicallyastringname,althoughnumbersmaybeused.

Creatingadictionaryissimplyamatterofassigningthekey:valuepairsasacomma-separatedlistbetweencurlybrackets(braces)toanameofyourchoice.Stringsmustbeenclosedwithinquotes,asusual,anda:coloncharactermustcomebetweenthekeyanditsassociatedvalue.

Inotherprogramminglanguagesalistisoftencalledan“array”andadictionaryisoftencalledan“associativearray”.

Akey:valuepaircanbedeletedfromadictionarybyspecifyingthedictionarynameandthepair’skeytothedelkeyword.Conversely,akey:valuepaircanbeaddedtoadictionarybyassigningavaluetothedictionary’snameandanewkey.

Pythondictionarieshaveakeys()methodthatcanbedot-suffixedtothedictionarynametoreturnalist,inrandomorder,ofallthekeysinthatdictionary.Ifyoupreferthekeystobesortedintoalphanumericorder,simplyenclosethestatementwithintheparenthesesofthePythonsorted()function.

AdictionarycanbesearchedtoseeifitcontainsaparticularkeywiththePythoninoperator,usingthesyntaxkeyindictionary.ThesearchwillreturnaBooleanTruevaluewhenthekeyisfoundinthespecifieddictionary,otherwiseitwillreturnFalse.

DictionariesarethefinaltypeofdatacontaineravailableinPythonprogramming.Insummary,thevarioustypesare:

• Variable–storesasinglevalue

• List–storesmultiplevaluesinanorderedindexarray

• Tuple–storesmultiplefixedvaluesinasequence

• Set–storesmultipleuniquevaluesinanunorderedcollection

• Dictionary–storesmultipleunorderedkey:valuepairs

Dataisfrequentlyassociatedaskey:valuepairs–forexample,whenyousubmitawebformatextvaluetypedintoaninputfieldistypicallyassociatedwiththattextfield’snameasitskey.

dict.py

Startanewprogrambyinitializingadictionarythendisplayitsdataclasstypeanditskey:valuecontentsinfo={‘name’:‘Bob’,‘ref’:‘Python’,‘sys’:‘Win’}

print(‘info:’,type(info))print(‘Dictionary:’,info)

Next,displayasinglevaluereferencedbyitskey

print(‘\nReference:’,info[‘ref’])

Now,displayallkeyswithinthedictionary

print(‘\nKeys:’,info.keys())

Deleteonepairfromthedictionaryandaddareplacementpairthendisplaythenewkey:valuecontentsdelinfo[‘name’]

info[‘user’]=‘Tom’print(‘\nDictionary:’,info)

Finally,searchthedictionaryforaspecifickeyanddisplaytheresultofthesearch

print(‘\nIsThereAnameKey?:’,’name’ininfo)

Savethefilethenrunthisprogram–toseethedictionarykeysandvalues

Noticethatquoteswithinastringmustbeprecededbyabackslashescapecharacter–topreventthestringbeingprematurelyterminated.

Summary• Multiplevariablescanbeinitializedinasinglestatementusingasequenceof=

assignments

• APythonlistisanarrayvariablethatcanstoremultipleitemsofdatainsequentiallynumberedelementsthatstartatzero

• Datastoredinalistelementcanbereferencedusingthelistnamefollowedbyanindexnumberin[]squarebrackets

• Alistelementcanhavemorethanoneindextorepresentmultipledimensions,suchasXandYcoordinates

• Listvariableshaveanumberofmethodsthatcanbedot-suffixedtothelistnameformanipulation

• Thelen()functionreturnsthelengthofaspecifiedlist

• AnindividuallistelementcanbedeletedbyspecifyingthelistnameandelementindexnumbertothePythondelkeyword

• APythontupleisanimmutablelistwhosevaluescanbeassignedtoindividualvariablesby“sequenceunpacking”

• Datastoredinatupleelementcanbereferencedusingthetuplenamefollowedbyanindexnumberin[]squarebrackets

• APythonsetisanorderedcollectionofuniqueelementswhosevaluescanbecomparedandmanipulatedbyitsmethods

• Datastoredinasetcannotbereferencedbyitsindexnumber

• Setvariableshavemethodsthatcanbedot-suffixedtothelistnameformanipulationandcomparison

• ThePythonbuilt-inmembershipinoperatorcanbeusedtoseekavaluewithinaset

• Asetcanbeconvertedtoaregularlistusingthelist()functiontoallowreferenceofelementdatabyindexnumber

• APythondictionaryisalistofkey:valuepairsofdatainwhicheachkeymustbeunique

• Datastoredinadictionaryelementcanbereferencedusingthedictionarynamefollowedbyitskeyin[]squarebrackets

5Controllingblocks

Thischapterdemonstrateshowtocreatecodetocontroltheflowofyourprograms.

BranchingchoicesCountingloopsLoopingconditionsSkippingloopsCatchingerrorsSummary

BranchingchoicesAsinmanyprogramminglanguagesthePythonifkeywordperformsabasicconditionaltestthatevaluatesagivenexpressionforaBooleanvalueofTrueorFalse.Thisallowsaprogramtoproceedindifferentdirectionsaccordingtotheresultofthetestandisknownas“conditionalbranching”.

InPython,thetestedexpressionmustbefollowedbya:colon,thenstatementstoexecutewhenthetestsucceedsshouldfollowbelowonseparatelinesandeachlinemustbeindentedfromtheiftestline.Thesizeoftheindentationisnotimportantbutitmustbethesameforeachline.Sothesyntaxlookslikethis:iftest-expression:

statements-to-execute-when-test-expression-is-Truestatements-to-execute-when-test-expression-is-True

Theif:elif:else:sequenceisthePythonequivalentoftheswitchorcasestatementsfoundinotherlanguages.

Optionally,aniftestcanofferalternativestatementstoexecutewhenthetestfailsbyappendinganelsekeywordafterthestatementstobeexecutedwhenthetestsucceeds.Theelsekeywordmustbefollowedbya:colonandalignedwiththeifkeywordbutitsstatementsmustbeindentedinalikewisemanner,soitssyntaxlookslikethis:iftest-expression:

statements-to-execute-when-test-expression-is-Truestatements-to-execute-when-test-expression-is-True

else:statements-to-execute-when-test-expression-is-Falsestatements-to-execute-when-test-expression-is-False

Aniftestblockcanbefollowedbyanalternativetestusingtheelifkeyword(“elseif”)thatoffersstatementstobeexecutedwhenthealternativetestsucceeds.This,too,mustbealignedwiththeifkeyword,followedbya:colon,anditsstatementsindented.Afinalelsekeywordcanthenbeaddedtoofferalternativestatementstoexecutewhenthetestfails.Thesyntaxforthecompleteif-elif-elsestructurelookslikethis:iftest-expression-1:

statements-to-execute-when-test-expression-1-is-Truestatements-to-execute-when-test-expression-1-is-True

eliftest-expression-2:

statements-to-execute-when-test-expression-2-is-Truestatements-to-execute-when-test-expression-2-is-True

else:

statements-to-execute-when-test-expressions-are-Falsestatements-to-execute-when-test-expressions-are-False

IndentationofcodeisveryimportantinPythonasitidentifiescodeblockstotheinterpreter–otherprogramminglanguagesusebracketingsuchas{}braces.

if.py

Startanewprogrambyinitializingavariablewithuserinputofanintegervalue

num=int(input(‘PleaseEnterANumber:‘))

Next,testthevariableanddisplayanappropriateresponse

ifnum>5:print(‘NumberExceeds5’)

elifnum<5:print(‘NumberisLessThan5’)

else:print(‘NumberIs5’)

Now,testthevariableagainusingtwoexpressionsanddisplayaresponseonlyuponsuccessifnum>7andnum<9:

print(‘Numberis8’)ifnum==1ornum==3:

print(‘NumberIs1or3’)

Savethenruntheprogram–toseeconditionalbranchinginaction

Theuserinputisreadasastringvaluebydefaultsomustbecastasanintdatatypewithint()forarithmeticalcomparison.

TheandkeywordensurestheevaluationisTrueonlywhenbothtestssucceed,whereastheorkeywordensurestheevaluationisTruewheneithertestsucceeds.

CountingloopsAsinotherprogramminglanguages,thePythonforkeywordloopsoverallitemsinanylistspecifiedtotheinkeyword.InPython,thisstatementmustendwitha:coloncharacterandstatementstobeexecutedoneachiterationoftheloopmustbeindented:foreach-iteminlist-name:

statements-to-execute-on-each-iterationstatements-to-execute-on-each-iteration

Becauseastringissimplyalistofcharacters,theforinstatementcanloopovereachcharacter.Similarly,aforinstatementcanloopovereachelementinalist,eachiteminatuple,eachmemberofaset,oreachkeyinadictionary.

TheforloopinPythonisunlikethatinotherlanguagessuchasCasitdoesnotallowstepsizeandendtobespecified.

Aforinloopiteratesovertheitemsofanylistorstringintheorderthattheyappearinthesequencebutyoucannotdirectlyspecifythenumberofiterationstomake,ahaltingcondition,orthesizeofiterationstep.Youcan,however,usethePythonrange()functiontoiterateoverasequenceofnumbersbyspecifyinganumericendvaluewithinitsparentheses.Thiswillgenerateasequencethatstartsatzeroandcontinuesupto,butnotincluding,thespecifiedendvalue.Forexample,range(5)generates0,1,2,3,4.

Optionally,youcanspecifybothastartandendvaluewithintheparenthesesoftherange()function,separatedbyacomma.Forexample,range(1,5)generates1,2,3,4.Also,youcanspecifyastartvalue,endvalue,andastepvaluetotherange()functionasacomma-separatedlistwithinitsparentheses.Forexample,range(1,14,4)generates1,5,9,13.

Youcanspecifythelist’snamewithintheparenthesesofPython’senumerate()functiontodisplayeachelement’sindexnumberanditsassociatedvalue.

Therange()functioncangenerateasequencethatdecreases,countingdown,aswellasthosethatcountupward.

Whenloopingthroughmultiplelistssimultaneously,theelementvaluesofthesameindexnumberineachlistcanbedisplayedtogetherbyspecifyingthelistnamesasacomma-separatedlistwithintheparenthesesofPython’szip()function.

Whenloopingthroughadictionary,youcandisplayeachkeyanditsassociatedvalueusingthedictionaryitems()methodandspecifyingtwocomma-separatedvariablenamestotheforkeyword–oneforthekeynameandtheotherforitsvalue.

for.py

Startanewprogrambyinitializingaregularlist,afixedtuplelist,andanassociativedictionarylistchars=[‘A’,‘B’,‘C’]

fruit=(‘Apple’,‘Banana’,‘Cherry’)info={‘name’:’Mike’,‘ref’:’Python’,‘sys’:’Win’}

Next,addstatementstodisplayalllistelementvalues

print(‘Elements:\t’,end=‘‘)

foriteminchars:print(item,end=‘‘)

Now,addstatementstodisplayalllistelementvaluesandtheirrelativeindexnumberprint(‘\nEnumerated:\t’,end=‘‘)

foriteminenumerate(chars):print(item,end=‘‘)

Then,addstatementstodisplayalllistandtupleelements

print(‘\nZipped:\t’,end=‘‘)

foriteminzip(chars,fruit):print(item,end=‘‘)

Finally,addstatementstodisplayalldictionarykeynamesandassociatedelementvaluesprint(‘\nPaired:’)

forkey,valueininfo.items():print(key,‘=’,value)

Savethenruntheprogram–toseetheitemsdisplayedbytheloopiterations

Inprogrammingterms,anythingthatcontainsmultipleitemsthatcanbeloopedoverisdescribedas“iterable”.

LoopingconditionsAloopisapieceofcodeinaprogramthatautomaticallyrepeats.Onecompleteexecutionofallstatementswithinaloopiscalledan“iteration”ora“pass”.Thelengthoftheloopiscontrolledbyaconditionaltestmadewithintheloop.WhilethetestedexpressionisfoundtobeTrue,theloopwillcontinue–untilthetestedexpressionisfoundtobeFalse,atwhichpointtheloopends.

InPythonprogramming,thewhilekeywordcreatesaloop.Itisfollowedbythetestexpressionthena:coloncharacter.Statementstoexecutewhenthetestsucceedsfollowbelowonseparatelines,eachlineindentedfromthewhiletestline.Importantly,theloopstatementblockmustincludeastatementthatwillchangetheresultofthetestexpressionevaluation–otherwiseaninfiniteloopiscreated.

UnlikeotherPythonkeywords,thekeywordsTrueandFalsebeginwithuppercaseletters.

IndentationofcodeblocksmustalsobeobservedinPython’sinteractivemode–likethisexamplethatproducesaFibonaccisequenceofnumbersfromawhileloop:

Loopscanbenested,onewithinanother,toallowcompleteexecutionofalliterationsofaninnernestedlooponeachiterationoftheouterloop.A“counter”variablecanbeinitializedwithastartingvalueimmediatelybeforeeachloopdefinition,includedinthetestexpression,andincrementedoneachiterationuntilthetestfails–atwhichpointtheloopends.

TheinteractivePythoninterpreterautomaticallyindentsandwaitswhenitexpectsfurthercodestatementsfromyou.

while.py

Startanewprogrambyinitializinga“counter”variableanddefineanouterloopusingthecountervariableinitstestexpressioni=1

whilei<4:

Next,addindentedstatementstodisplaythecounter’svalueandincrementitsvalueoneachiterationoftheloop

print(‘OuterLoopIteration:’,i)i+=1

Now,(stillindented)initializeasecond“counter”variableanddefineaninnerloopusingthisvariableinitstestexpression

j=1

whilej<4:

Finally,addfurther-indentedstatementstodisplaythiscounter’svalueandincrementitsvalueoneachiteration

print(‘\tInnerLoopIteration:‘,j)j+=1

Savethenrunthisprogram–toseetheoutputdisplayedoneachloopiteration

Theoutputprintedfromtheinnerloopisindentedfromthatoftheouterloopbythe\ttabcharacter.

The+=assignmentstatementi+=1issimplyashorthandwaytosayi=i+1–youcanalsouse*=/=-=shorthandtoassignvaluestovariables.

SkippingloopsThePythonbreakkeywordcanbeusedtoprematurelyterminatealoopwhenaspecifiedconditionismet.Thebreakstatementissituatedinsidetheloopstatementblockandisprecededbyatestexpression.WhenthetestreturnsTrue,theloopendsimmediatelyandtheprogramproceedsontothenexttask.Forexample,inanestedinnerloopitproceedstothenextiterationoftheouterloop.

nest.py

Startanewprogramwithastatementcreatingaloopthatiteratesthreetimes

foriinrange(1,4):

Next,addanindentedstatementcreatinga“nested”innerloopthatalsoiteratesthreetimes

forjinrange(1,4):

Now,addafurther-indentedstatementintheinnerlooptodisplaythecounternumbers(ofboththeouterloopandtheinnerloop)oneachiterationoftheinnerloop

print(‘Runningi=’+i+‘j=’+j)

Savethenrunthisprogram–toseethecountervaluesoneachloopiteration

Comparethesenestedforloopswiththenestedwhileloopsexamplehere.

break.py

Insertabreakstatementatthestartoftheinnerlooptobreakfromthatloop–thenruntheprogramagain

ifi==2andj==1:

print(‘Breaksinnerloopati=2j=1’)break

ThePythoncontinuekeywordcanbeusedtoskipasingleiterationofaloopwhenaspecifiedconditionismet.Thecontinuestatementissituatedinsidetheloopstatementblockandisprecededbyatestexpression.WhenthetestreturnsTrue,thatoneiterationendsandtheprogramproceedstothenextiteration.

Now,insertacontinuestatementatthestartoftheinnerloopblocktoskipthefirstiterationofthatloop–thenruntheprogramoncemore

ifi==1andj==1:

print(‘Continuesinnerloopati=1j=1’)continue

Thebreakstatementhaltsallthreeiterationsoftheinnerloopwhentheouterlooprunsitforthesecondtime.

continue.py

Thecontinuestatementskipsthefirstiterationoftheinnerloopwhentheouterloopfirstrunsit.

CatchingerrorsSectionsofaprograminwhichitispossibletoanticipateerrors,suchasthosehandlinguserinput,cantypicallybeenclosedinatryexceptblocktohandle“exceptionerrors”.Thestatementstobeexecutedaregroupedinatry:blockandexceptionsarepassedtotheensuingexcept:blockforhandling.Optionally,thismaybefollowedbyafinally:blockcontainingstatementstobeexecutedafterexceptionshavebeenhandled.

Pythonrecognizesmanybuilt-inexceptionssuchastheNameErrorwhichoccurswhenavariablenameisnotfound,theIndexErrorwhichoccurswhentryingtoaddressanon-existentlistindex,andtheValueErrorwhichoccurswhenabuilt-inoperationorfunctionreceivesaninappropriatevalue.

Eachexceptionreturnsadescriptivemessagethatcanusefullybeassignedtoavariablewiththeaskeyword.Thiscanthenbeusedtodisplaythenatureoftheexceptionwhenitoccurs.

try.py

Startanewprogrambyinitializingavariablewithastringvalue

title=‘CodingforBeginnersInEasySteps’

Next,addatrystatementblockthatattemptstodisplaythevariablevalue–butspecifiesthenameincorrectlytry:

print(titel)

Now,addanexceptstatementblocktodisplayanerrormessagewhenaNameError

occursexceptNameErrorasmsg:

print(msg)

Savethenruntheprogram–toseehowtheerrorgetshandled

Insomeprogramminglanguagesthisstructureisknownastry-catchexceptionhandling.

Multipleexceptionscanbehandledbyspecifiyingtheirtypeasacomma-separatedlistinparentheseswithintheexceptblock:except(NameError,IndexError)asmsg:

print(msg)

Youcanalsocompeltheinterpretertoreportanexceptionbyusingtheraisekeywordtospecifythetypeofexceptiontoberecognizedandacustomdescriptivemessageinparentheses.

raise.py

StartanewPythonscriptbyinitializingavariablewithanintegervalue

day=32

Next,addatrystatementblockthatteststhevariablevaluethenspecifiesanexceptionandcustommessagetry:

ifday>31:

raiseValueError(‘InvalidDayNumber’)#Morestatementstoexecutegetaddedhere.

Now,addanexceptstatementblocktodisplayanerrormessagewhenaValueErroroccursexceptValueErrorasmsg:

print(‘TheProgramfoundAn’,msg)

Then,addafinallystatementblocktodisplayamessageaftertheexceptionhasbeenhandledsuccesfullyfinally:

print(‘ButTodayIsBeautifulAnyway.’)

Savethenruntheprogram–toseetheraisederrorgethandled

Statementsinthetryblockareallexecutedunlessoruntilanexceptionoccurs.

Summary• TheifkeywordperformsaconditionaltestonanexpressionforaBooleanvalueofTrue

orFalse

• Conditionalbranchingprovidesalternativestoaniftestwiththeelseandelifkeywords

• Pythontestedexpressionsmustbefollowedbya:coloncharacterandstatementblocksbeindentedfromthetestline

• Aforinloopiteratesovereachiteminaspecifiedlistorstring

• Pythonforinloopstatementsmustbefollowedbya:coloncharacterandstatementblocksbeindentedfromthestatement

• Therange()functiongeneratesanumericalsequencethatcanbeusedtospecifythelengthofaforinloop

• Theenumerate()functioncanspecifythenameofalisttodisplayeachelementindexnumberanditsstoredvalue

• Thezip()functioncandisplaythestoredvaluesofthesameindexnumberinmultiplelists

• AwhilelooprepeatswhileaninitialtestexpressionremainsTrueandendswhenthattestexpressionbecomesFalse

• Pythonwhileloopstatementsmustbefollowedbya:coloncharacterandstatementblocksbeindentedfromthestatement

• Thevalueofacountervariablecanbetestedinatestexpressiontocontrolthenumberofloopiterations

• ThebreakkeywordcanbeusedtotestanexpressionforaTruevalueandimmediatelyexitaloopwhenthetestreturnsFalse

• ThecontinuekeywordcanbeusedtotestanexpressionforaTruevalueandexitasingleiterationwhenthetestreturnsFalse

• Anticipatedruntimeexceptionerrorscanbehandledbyenclosingstatementsinatryexceptblock

• Optionally,afinallystatementcanbeusedtospecifystatementstobeexecutedafterexceptionshavebeenhandled

• Pythontry,except,andfinallystatementsmustbefollowedbya:colonandstatementblocksbeindentedfromthestatement

6Creatingfunctions

Thischapterdemonstrateshowtocodere-usableblocksofcodeinyourprograms.

DefiningblocksAddingparametersReturningresultsStoringfunctionsImportingfunctionsSummary

DefiningblocksPreviousexamplesinthisbookhaveusedbuilt-infunctionsofthePythonprogramminglanguage,suchastheprint()function.However,mostprogramshaveanumberofcoder-definedcustomfunctionsthatcanbecalledasrequiredwhentheprogramruns.

AcustomPythonfunctioniscreatedusingthedef(definition)keywordfollowedbyanameofyourchoiceand()parentheses.Thecodercanchooseanynameforafunctionexceptthekeywordsoftheprogramminglanguageandthenameofanexistingbuilt-infunction.Thislinemustendwitha:coloncharacter,thenthestatementstobeexecutedwheneverthefunctiongetscalledmustappearonlinesbelowandindented.Syntaxofafunctiondefinition,therefore,lookslikethis:deffunction-name():

statements-to-be-executedstatements-to-be-executed

FunctionstatementsmustbeindentedfromthedefinitionlinebythesameamountsothePythoninterpretercanrecognizetheblock.

Oncethefunctionstatementshavebeenexecuted,programflowresumesatthepointdirectlyfollowingthefunctioncall.Thismodularityisveryusefulinprogrammingtoisolatesetroutinessotheycanbecalleduponrepeatedly.

Tocreatecustomfunctionsitisnecessarytounderstandtheaccessibility(“scope”)ofvariablesinaprogram:

• Variablescreatedoutsidefunctionscanbereferencedbystatementsinsidefunctions–theyhave“global”scope

• Variablescreatedinsidefunctionscannotbereferencedfromoutsidethefunctioninwhichtheyhavebeencreated–thesehave“local”scope

Thelimitedaccessibilityoflocalvariablesmeansthatvariablesofthesamenamecanappearindifferentfunctionswithoutconflict.

Avoidusingglobalvariablesinordertopreventaccidentalconflict–useonlylocalvariableswherepossible.

Ifyouwanttocoercealocalvariabletomakeitaccessibleelsewhereitmustfirstbe

declaredwiththePythonglobalkeywordfollowedbyitsnameonly.Itmaysubsequentlybeassignedavaluethatcanbereferencedfromanywhereintheprogram.Whereaglobalvariableandalocalvariablehavethesamename,thefunctionwillusethelocalversion.

def.py

Startanewprogrambyinitalizingaglobalvariable

global_var=1

Next,createafunctionnamed“my_vars”todisplaythevaluecontainedwithintheglobalvariabledefmy_vars():

print(‘Globalvariable:’,global_var)

Now,addindentedstatementstothefunctionblocktoinitializealocalvariableanddisplaythevalueitcontains

local_var=2print(‘Localvariable:’,local_var)

Then,addindentedstatementstothefunctionblocktocreateacoercedglobalvariableandassignaninitialvalue

globalinner_var

inner_var=3

Addastatementafterthefunctiontocalluponthatfunctiontoexecutethestatementsitcontainsmy_vars()

Finally,addastatementtodisplaythevaluecontainedinthecoercedglobalvariableprint(‘CoercedGlobal:’,inner_var)

Savethenruntheprogram–toseethecustomfunctiondisplaythevariablevalues

Variablesthatarenotglobalbutappearinsomeouterscopecanbeaddressedusingthenonlocalkeyword.

AddingparametersWhendefiningacustomfunctioninaprogramyoumayoptionallyspecifya“parameter”namebetweenthefunction’sparentheses.An“argument”valuecanthenbepassedtothatparameterbyspecifyingthevalueintheparenthesesofthecalltothefunction.Thefunctioncannowusethatpassedinvalueduringitsexecutionbyreferencingitviatheparametername.Forexample,definingafunctiontoacceptaparametertoprintout:defecho(user):

print(‘User:’,user)

Parametersarespecialvariablesforinternaluseonlywithinafunction–theymustadheretothesamenamingrulesasregularvariables.

Acalltothisfunctionmustspecifyanargumentvaluetobepassedtotheparameterinitsparenthesessoitcanbeprintedout:echo(‘Mike’)

Multipleparameterscanbespecifiedinthefunctiondefinitionbyincludingacomma-separatedlistofparameternameswithinthefunctionparentheses:defecho(user,lang,sys):

print(User:’,user,‘Language:’,lang,‘Platform:’,sys)

Whencallingafunctionwhosedefinitionspecifiesparameters,thecallmustincludethesamenumberofargumentsasparameters.Forexample,tocallthisexamplewithmultipleparameters:echo(‘Mike’,‘Python’,‘Windows’)

Thepassedvaluesmustappearinthesameorderastheparameterlistunlessthecalleralsospecifiestheparameternameslikethis:echo(lang=‘Python’,user=‘Mike’,sys=‘Windows’)

Optionally,adefaultvaluemaybespecifiedintheparameterlistwhendefiningafunction.Thiswillbeoverriddenwhenthecallerspecifiesavalueforthatparameterbutwillbeusedbythefunctionwhennoargumentvaluegetspassedbythecaller:defecho(user,lang,sys=‘Linux’):

print(User:’,user,‘Language:’,lang,‘Platform:’,sys)

Thismeansyoumaycallthefunctionpassingfewervaluesthanthenumberofparametersspecifiedinthefunctiondefinition,tousethedefaultparametervalue,orpassthesamenumberofvaluesasspecifiedparameterstooverridethedefaultvalue.

Nameparametersthesameasvariablespassedtothemtomakethedatamovementobvious.

param.py

Startanewprogrambydefiningafunctionwiththreeparametersthatwillprintoutpassed-inargumentvaluesdefecho(user,lang,sys):

print(‘User:’,user,‘Language:’,lang,‘Platform:’,sys)

Next,callthefunctionpassingstringargumentvaluestothefunctionparametersintheordertheyappearecho(‘Mike’,‘Python’,‘Windows’)

Now,callthefunctionpassingstringargumentstothefunctionparametersbyspecifyingtheparameternamesecho(lang=‘Python’,sys=‘MacOS’,user=‘Anne’)

Then,defineanotherfunctionwithtwoparametershavingdefaultvaluesthatwillprintoutparametervaluesdefmirror(user=‘Carole’,lang=‘Python’):

print(‘\nUser:’,user,‘Language:’,lang)

Finally,addstatementstocallthesecondfunctionbothusingandoverridingdefaultvaluesmirror()

mirror(lang=‘Java’)mirror(user=‘Tony’)

mirror(‘Susan’,‘C++’)

Savethenruntheprogram–toseethefunctiondisplaytheargumentvaluesordefaultparametervalues

Argumentsaretheactualdatavaluespassedtofunctionparametersbythefunctioncall.

ReturningresultsLikePython’sbuilt-instr()function,whichreturnsastringrepresentationofthevaluespecifiedasitsargumentbythecaller,yourcustomfunctionscanalsoreturnavaluetotheircallerbyusingthereturnkeywordtospecifyavaluetobereturned.Forexample,toreturntothecallerthetotalofaddingtwospecifiedparametervalueslikethis:defsum(a,b):

returna+b

Thereturnedresultmaybeassignedtoavariablebythecallerforsubsequentusebytheprogramlikethis:total=sum(8,4)print(‘EightPlusFourIs:’,total)

Orthereturnedresultmaybeuseddirectly“in-line”likethis:print(‘EightPlusFourIs:’,sum(8,4))

Typically,areturnstatementwillappearattheveryendofafunctionblocktoreturnthefinalresultofexecutingallstatementscontainedinthatfunction.

Areturnstatementmay,however,appearearlierinthefunctionblocktohaltexecutionofallsubsequentstatementsinthatblock.Thisimmediatelyresumesexecutionoftheprogramatthecaller.Optionally,thereturnstatementmayspecifyavaluetobereturnedtothecallerorthevaluemaybeomitted.Wherenovalueisspecified,adefaultvalueofNoneisassumed.Typically,thisisusedtohaltexecutionofthefunctionstatementsafteraconditionaltestisfoundtobeFalse.Forexample,whereapassedargumentvalueisbelowaspecifiednumber:defsum(a,b):

ifa<5:return

returna+b

Youcanspecifyadefaultvalueforaparameterinthefunctiondefinition.

Inthiscase,thefunctionwillreturnthedefaultvalueNonewhenthefirstpassedargumentvalueisbelowfiveandthefinalstatementwillnotbeexecuted.

Wherethefunctionistoperformarithmetic,userinputcanbevalidatedforintegervalueswiththebuilt-inisdigit()function.

return.py

Startanewprogrambyinitializingavariablewithuserinputofanintegervalueformanipulationnum=input(‘EnterAnInteger:’)

Next,addafunctiondefinitionthatacceptsasingleargumentvaluetobepassedfromthecallerdefsquare(num):

Now,insertintothefunctionblockanindentedstatementtovalidatethepassedargumentvalueasanintegerorhaltfurtherexecutionofthefunction’sstatements

ifnotnum.isdigit():

return‘InvalidEntry’

Then,addindentedstatementstocastthepassedargumentvalueasanintdatatypethenreturnthesumofsquaringthatvaluetothecaller

num=int(num)returnnum*num

Finally,addastatementtooutputastringandthereturnedvaluefromthefunctioncallprint(num,‘SquaredIs:’,square(num))

Savethenruntheprogram–toseethefunctiondisplaythereturnedvalues

Rememberthatuserinputisreadasastrdatatype–somustbecastintoanint

orfloatdatatypeforarithmetic.

StoringfunctionsFunctiondefinitionscanusefullybestoredinoneormoreseparatefilesforeasiermaintenanceandtoallowthemtobeusedinseveralprogramswithoutcopyingthedefinitionsintoeachone.EachPythonfilestoringfunctiondefinitionsiscalleda“module”andthemodulenameisthefilenamewithoutthe“.py”extension.

FunctionsstoredinthemodulearemadeavailabletoaprogramusingthePythonimportkeywordfollowedbythemodulename.Althoughnotessential,itiscustomarytoputanyimportstatementsatthebeginningoftheprogram.

Importedfunctionscanbecalledusingtheirnamedotsuffixedafterthemodulename.Forexample,a“steps”functionfromanimportedmodulenamed“ineasy”canbecalledwithineasy.steps().

Wherefunctionsstoredinamoduleincludeparameters,itisoftenusefultoassignadefaultvaluetotheparameterinthedefinition.Thismakesthefunctionmoreversatileasitbecomesoptionalforthecalltospecifyaparametervalue.

cat.py

Startanewmodulebydefiningafunctionthatsuppliesadefaultstringvaluetoitsparameterfordisplaydefpurr(pet=‘ACat’):

print(pet,‘SaysMEOW!’)

Next,addtwomorefunctiondefinitionsthatalsosupplydefaultstringvaluestotheirparametersfordisplaydeflick(pet=‘ACat’):

print(pet,‘DrinksMilk’)

defnap(pet=‘ACat’):print(pet,‘SleepsByTheFire’)

Now,savethefileas“cat.py”sothemoduleisnamed“cat”

Startanewprogramwithastatementtomakethe“cat”modulefunctionsavailable

importcat

kitty.py

Next,calleachfunctionwithoutsupplyinganargument

cat.purr()

cat.lick()cat.nap()

Now,calleachfunctionagainandpassanargumenttoeachthensavethefile

cat.purr(‘Kitty’)

cat.lick(‘Kitty’)cat.nap(‘Kitty’)

Startanotherprogrambymakingthe“cat”modulefunctionsavailableoncemore

importcat

Thenrequesttheuserentersanametooverwritethedefaultparametervalue

pet=input(‘EnterAPetName:‘)

Finally,calleachfunctionpassingtheuser-definedvalueastheargument

cat.purr(pet)

cat.lick(pet)cat.nap(pet)

Savethenruntheseprograms–toseeoutputfromtheimportedmoduleineachprogram

tiger.py

Youcancreateanaliaswhenimportingamoduleusingimportaskeywords.Forexample,importcatastomallowsyoutousetomasthefunctionprefixincalls.

ImportingfunctionsInternally,eachPythonmoduleandprogramhasitsown“symboltable”whichisusedbyallfunctionsdefinedinthatcontextonly.Thisavoidspossibleconflictswithfunctionsofthesamenameinanothermoduleifbothmoduleswereimportedintooneprogram.

Whenyouimportamodulewithanimportstatement,thatmodule’ssymboltabledoesnotgetaddedtotheprogram’ssymboltable–onlythemodule’snamegetsadded.Thatiswhyyouneedtocallthemodule’sfunctionsusingtheirmodulenameprefix.Importinga“steps”functionfromamodulenamed“ineasy”andanother“steps”functionfromamodulenamed“dance”meanstheycanbecalledwithoutconflictasineasy.steps()anddance.steps().

Whereyouimportindividualfunctionnames,themodulenamedoesnotgetimported–soitcannotbeusedasaprefix.

Generally,itispreferabletoavoidconflictsbyimportingthemodulenameandcallingitsfunctionswiththemodulenameprefix,butyoucanimportindividualfunctionnamesinsteadwithafromimportstatement.Themodulenameisspecifiedafterthefromkeyword,andfunctionstoimportarespecifiedasacomma-separatedlistaftertheimportkeyword.Alternatively,the*wildcardcharactercanbespecifiedafterimporttoimportallfunctionnamesintotheprogram’sownsymboltable.Thismeansthefunctionscanbecalledwithoutamodulenameprefix.

dog.py

Startanewmodulebydefiningafunctionthatsuppliesadefaultstringvaluetoitsparameterdefbark(pet=‘ADog’):

print(pet,‘SaysWOOF!’)

Next,addtwomorefunctiondefinitionsthatalsosupplydefaultstringvaluestotheirparametersdeflick(pet=‘ADog’):

print(pet,‘Drinkswater’)

defnap(pet=‘ADog’):print(pet,‘SleepsInTheSun’)

Savethefileas“dog.py”sothemoduleisnamed“dog”

pooch.py

Startanewprogramwithastatementtomakeindividual“dog”modulefunctionsavailablefromdogimportbark,lick,nap

Next,calleachfunctionwithoutsupplyinganargument

bark()

lick()nap()

Now,calleachfunctionagainandpassanargumentvaluetoeachthensavethefile

bark(‘Pooch’)

lick(‘Pooch’)nap(‘Pooch’)

Startanotherprogrambymakingall“dog”modulefunctionsavailable

fromdogimport*

Requestauserentrytooverwritethedefaultparameter

pet=input(‘EnterAPetName:‘)

Finally,calleachfunctionpassingtheuser-definedvalueastheargument

bark(pet)

lick(pet)nap(pet)

Savethenruntheseprograms–toseeoutputfromtheimportedfunctions

fido.py

Forlargerprogramsyoucanimportmodulesintoothermodulestobuildamodulehierarchy.

Summary• Functionsaredefinedusingthedefkeywordfollowedbyanameofyourchoiceand()

parentheses

• Afunctiondefinitionlinemustendwitha:coloncharacteranditsblockofstatementstoexecutewhenthefunctiongetscalledmustbeindentedbelowthatline

• Variableswithglobalscopecanbereferencedfromanywherewithinthatprogram

• Variableswithlocalscopecanonlybereferencedfromwithinthefunctioninwhichtheyaredeclared

• Alocalvariablecanbecoercedtomakeitgloballyaccessiblebyfirstdeclaringitusingtheglobalkeyword

• Functionparametersarespecialvariablesforuseonlywithinafunction,andargumentsaredatavaluespassedtoparameters

• Parametersaredeclaredasacomma-separatedlistwithintheparenthesesofafunctiondefinition

• Functioncallsmustsupplyargumentdataforeachfunctionparameterunlessadefaultvalueisspecifiedintheirdeclaration

• Datapassedtoparametersinafunctioncallmustappearinthesameorderastheparametersunlesstheirnamesarespecified

• Optionally,thereturnkeywordcanbeusedwithinafunctiontoreturnavaluetothecaller

• Functionscanbestoredinmodulesthatarenamedasthefilenamewithoutthe“.py”fileextension

• Animportstatementmakesmodulefunctionsavailableinaprogrambydot-suffixingtheirnameafterthemodulename

• Internally,eachPythonmodulehasitsownsymboltablesolike-namedfunctionsindifferentmodulesdonotconflict

• Afromimportstatementmakesmodulefunctionsavailableinaprogramwithouttheneedtodot-suffixtheirname

• Animport*statementcanbeusedtoimportamodule’sfunctionsintotheprogram’sownsymboltable

7Sortingalgorithms

Thischapterdemonstrateshowtocodeavarietyofsortingalgorithminstructionsequences.

CopyingsortsSelectingsortsInsertingsortsBubblingsortsMergingsortsPartitioningsortsSummary

CopyingsortsAn“algorithm”isawell-definedsequenceofinstructionstoperformaspecifictask.Eachalgorithmtakesoneormorevaluesasinputandproducesoneormoreresultingvaluesasoutput.

Analgorithmmaybecreatedincodeasafunctionwhosestatementsdefineasequenceofinstructionstoperformthetask.Inputvaluescanbepassedasargumentsinthefunctioncallandresultingvaluescanbereturnedasoutputfromthefunction.Algorithmscanbecoded,usingdatastructuresandcontrolstructuresinavarietyofways,toperformtaskssuchassortinglistsintoorder.

Pythonprovidesthesort()methodforlists(describedhere)butexamplesinthischapterdemonstratehowtocodevarioussortingalgorithmsthatarefoundinmanyprogramminglanguages.

Sortingefficiencymaydependuponthenatureofthelisttobesortedsodifferentalgorithmsmaybebest-suitedtoparticulartasks.Wherethetaskrequiresalisttobesorted,whiletheoriginalunsortedlistremainsintact,analgorithmfunctioncanbepassedareferencetothelistinanargumentasinput.Thefunctioncanthenmakeacopyoftheoriginallist,sortelementsofthatcopyintoorder,thenreturnthesortedcopylistasoutput.Thisalgorithmsimplycopieselementvaluesfromtheoriginallistintoanewlistarraythenarrangestheminascendingvalueorder.

Arraylistelementsarenumberedfromzero.Sohere,element[0]contains5,element[1]contains3,andsoon.

copy.py

Startanewprogrambydeclaringafunctiontoreceivealistreferenceasinputandreturnasortedcopyasoutputdefcopy_sort(array):

copy=array[:]sorted_copy=[]

#Algorithmsequencetobeaddedhere.returnsorted_copy

Next,addtheindentedalgorithmsequencetoinsertthecopiedelementvaluesintotheemptylistinorder

whilelen(copy)>0:

minimum=0forelementinrange(0,len(copy)):

ifcopy[element]<copy[minimum]:minimum=element

print(‘\tRemovingvalue’,copy[minimum],\‘from’,copy)

sorted_copy.append(copy.pop(minimum))

Now,addstatementstocreateanddisplayanunsortedlist

array=[5,3,1,2,6,4]print(‘CopySort…\nArray:’,array)

Finally,addstatementstodisplaytheunsortedlistanditssortedcopythensaveandruntheprogram–toseetheoriginallistremainsintactinunsortedorderprint(‘Copy:’,copy_sort(array))

print(‘Array:’,array)

Eachvalueispoppedoffthelistcopyinsequence–tobuildthesortedversionbyappendingeachtotheemptyarray.

SelectingsortsOftenyouwillwanttosorttheelementsofanunsortedlistarray“inplace”ratherthansortacopyoftheoriginallist,asdemonstratedinthepreviousexample.Thereareseveralpopularalgorithmsyoucanemploytosortarraysinplace,eachusingadifferenttechniquewiththeirownstrengthsandweaknesses.

A“selectionsort”algorithmexamineseachelementintheunsortedpartofanarraylistandselectstheelementcontainingthelowestvalue.Itthenswapstheselectedelementvaluewiththatcontainedintheelementatthebeginningoftheunsortedpartofthearraylist–therebyincreasingthesizeofthesortedpartofthearrayanddecreasingitsunsortedpart.Thisprocessisrepeateduntilallelementvaluesaresortedintoascendingorder.

Selectionsortisasimpleswap-basedalgorithmthatisrelativelyeasytounderstandandcodeasafunctionalgorithm.Itisoneofthetwomostefficientalgorithmsforsortingsmallarraysof20orsoelements.

Inthisexample,thefinalelementwillalreadycontainthehighestvaluewhenthepenultimateelementhasbeensorted.

selection.py

Startanewprogrambydeclaringafunctiontoreceivealistreferenceasinputandbeginalooptostoreeachelement’svalueandcurrentindexnumberdefselection_sort(array):

forindexinrange(0,len(array)-1):

value=array[index]current=index

#Algorithmsequencetobeaddedhere.

Next,addthealgorithmsequencetorepeatedlyswapthesmallestunsortedvaluewiththefirstunsortedvalue

forelementinrange(index+1,len(array)):ifarray[element]<array[current]:

current=elementarray[index]=array[current]

array[current]=valueprint(‘\tResolvingelement[‘,index,‘]to‘,array)

Now,addstatementstocreateanddisplayanunsortedlist

array=[5,3,1,2,6,4]

print(‘SelectionSort…\nArray:’,array)

Finally,addstatementstocallthealgorithmfunctionanddisplaythelistoncemore–toseethelistsortedinplaceselection_sort(array)print(‘Array:’,array)

Sortinginplaceswapstheelementvaluescontainedintheoriginalreferencedarraylist.

InsertingsortsThetechniqueofswappingelementswithaselectionsortalgorithm,demonstratedinthepreviousexample,workswellbutanalternativetechniquecanbeemployedtosimplyinsertelementsintoanarraylistatthecorrectascendingorderposition.

An“insertionsort”algorithmexaminesthenextelementintheunsortedpartofanarraylistand,ifrequired,insertstheelementatthecorrectascendingorderpositioninthearraylist.Toaccommodatetheinsertedelement,allotherelementsintheunsortedpartofthelistshifttotheright–increasingthesizeofthesortedpartofthearrayanddecreasingitsunsortedpart.Thisprocessisrepeatedforeachelementinturnuntilallelementvaluesaresortedintoascendingorder.

Insertionsortisasimplealgorithmthatisrelativelyeasytounderstandandcodeasafunctionalgorithm.Alongwiththeselectionsortalgorithmitisoneofthetwomostefficientalgorithmsforsortingsmallarraysof20orsoelements.Typically,insertionsortwillrequirefewercomparisonsthanaselectionsortsoisoftenseenasthebestmethodforsortingsmallarrays.

Inthisexample,twostepsareneededtogetthelowestvalueintotheveryfirstelement–as3islessthan5,then1islessthan3.

insertion.py

Startanewprogrambydeclaringafunctiontoreceivealistreferenceasinputandbeginalooptostorethecurrentelement’svaluedefinsertion_sort(array):

forindexinrange(1,len(array)):

value=array[index]#Algorithmsequencetobeaddedhere.

print(‘\tResolvingelement[‘,index,‘]to‘,array)

Next,addthealgorithmsequencetorepeatedlyinsertthecurrentvalueifsmallerthanthatinthecurrentelement

whilearray[index-1]>valueandindex>=1:array[index]=array[index-1]

index-=1array[index]=value

Now,addstatementstocreateanddisplayanunsortedlist

array=[5,3,1,2,6,4]

print(‘InsertionSort…\nArray:’,array)

Finally,addstatementstocallthealgorithmfunctionanddisplaythelistoncemore–toseethelistsortedinplaceinsertion_sort(array)print(‘Array:’,array)

Onsomeiterationsthisalgorithmrecognizesthatelementsinthe“unsorted”partarealreadysortedfollowingearlierinsertions.

BubblingsortsA“bubblesort”algorithmisafurthersimplealternativetotheselectionandinsertionsorttechniques.Thisalgorithmrepeatedlyexamineseachadjacentpairofelementsinanarraylistand,ifrequired,swapsthemaroundtoplacealowervaluebeforeahighervalue–untilallelementsaresortedintoascendingorder.

Bubblesortisasimplealgorithmthatisveryeasytounderstandandcodeasafunctionalgorithm.Althoughthebubblesorttechniqueisgenerallylessefficientthaninsertionsortorselectionsort,itisoneofthequickestalgorithmsfornearly-sortedarrays.

Inthisexample,noswapisneededonthefirstpassforelement[3]and[4]astheirvalues5and6areincorrectorder.

bubble.py

Startanewprogrambydeclaringafunctiontoreceivealistreferenceasinputandbeginanouterlooptorepeatedlyiteratethroughthearraylistdefbubble_sort(array):

forindexinrange(len(array)):#Algorithmsequencetobeaddedhere.

Next,addthealgorithmsequencetoiteratethroughthearraylistelements,upto

thepenultimateelement,andswapvaluesifthenextisgreaterthanthecurrentvalue

forelementinrange(len(array-1)-index):

ifarray[element]>array[element+1]:array[element],array[element+1]=\

array[element+1],array[element]print(‘\tResolvingelement[‘,element,\

‘]to‘,array)

Now,addstatementstocreateanddisplayanunsortedlist

array=[5,3,1,2,6,4]print(‘BubbleSort…\nArray:’,array)

Finally,addstatementstocallthealgorithmfunctionanddisplaythelistoncemore–toseethelistsortedinplacebubble_sort(array)

print(‘Array:’,array)

Inaveragesimplecases,insertionsortoutperformsselectionsort,andselectionsortoutperformsbubblesort.

MergingsortsWhilethesimpleselectionsort,insertionsort,andbubblesorttechniquesworkwellonsmallarraylists,moreefficientcomplexsortingalgorithmsgenerallyworkbetteronlargerarraylists.Typically,thesecomplexsortingalgorithmsemploya“divideandconquer”strategytofirstrepeatedlydividethelistintosub-sections,thenre-assemblethosesub-sectionsinsortedorder.

A“mergesort”algorithmisacomplexsortingalgorithmthatfirstrepeatedlydividesanarrayintoleftandrightsub-sectionsfromthearray’smid-pointuntilitisemptyorhasjustoneelement.Oncethedivisioniscomplete,allsub-sectionsareindividualelements.Thealgorithmthenmergesalltheindividualelementsintoasinglesortedlist.

Mergesortisacomplexalgorithmthatcanbecodedasafunctionalgorithm,whichmakes“recursive”callstorepeatedlydivideanarray.Mergesortisafastalgorithmthatonlycompareselementvalueswhenmergingtheelementsbackintoasortedarraylist.

Inthisexample,thearrayisonlysmallbutthemergesortalgorithmefficientlydividesthenre-assembleslargearraysinexactlythesameway.

Startanewprogrambydeclaringafunctiontoreceivealistreferenceandrepeatedlyiteratethroughanarraylistdefmerge_sort(array):

iflen(array)>1:middle=int(len(array)/2)

left=array[0:middle];right=array[middle:]print(‘\tSplitto’,left,right)

merge_sort(left);merge_sort(right)

#Algorithmsequencetobeaddedhere.

merge.py

Next,addthealgorithmsequencetodividethearraylistelementsintosub-sectionsthenmergethemtogether

i=j=0forelementinrange(len(array)):

L=left[i]ifi<len(left)elseNoneR=right[j]ifj<len(right)elseNone

if((LandR)and(L<R))orRisNone:array[element]=L;i+=1

elif((LandR)and(L>=R))orLisNone:array[element]=R;j+=1

print(‘\t\tMerging’,left,right)

Now,addstatementstocreateanddisplaythearraylist

array=[5,3,1,2,6,4]print(‘MergeSort…\nArray:’,array)

merge_sort(array)print(‘Array:’,array)

Thecodelistedhereusesasemi-colon;separatortowritetwostatementsonsomelines–dueonlytopagelimitations.

PartitioningsortsThetechniqueofdividinganarraylistintosub-sectionsaroundthearray’smid-pointwithamergesortalgorithm,demonstratedinthepreviousexample,workswellbutanalternativetechniquecanbeemployedtopartitionanarraylistarounda“pivot”point.

A“quicksort”algorithmisacomplexsortingalgorithmthatalsoemploysthe“divideandconquer”strategy.Thisfirstspecifiesaparticulararrayelementwhosevaluewillactasthepivot.Thealgorithmthenrepeatedlydividesthearrayintotwo“partitions”–onepartitioncontainingvalueslessthanthepivotandtheotherpartitioncontainingvaluesmorethanthepivot.Oncethepartitionoperationiscomplete,thefinalpivotisinitscorrectpositionsothealgorithmthenmergesthelesserpartitionwiththepivotandthegreaterpartitionintoasinglesortedlist.

Quicksortisacomplexalgorithmthatcanbecodedasafunctionalgorithm,whichmakes“recursive”callstorepeatedlydivideanarray.Quicksortisafastalgorithmthatcompareselementvalueswhendividingtheelementsintopartitions.

Inthisexamplethearrayisonlysmallbutthequicksortalgorithmefficientlypartitionsthenre-assembleslargearraysinexactlythesameway.

Opinionsvaryastowhichelementisbesttochooseasthepivotinthequicksortalgorithm.Somecodersliketochooseamiddleelement,asinthemergesortalgorithminthepreviousexample.Othersprefertochoosethefirstorlastelement,oranelementatsomearbitrarypositioninbetween–liketheexampleopposite.

Startanewprogrambydeclaringafunctiontoreceivealistreferenceandrepeatedlyiteratethroughanarraylistdefquick_sort(array):

iflen(array)>1:pivot=int(len(array)-1)

less=[];more=[]#Algorithmsequencetobeaddedhere.

quick_sort(less);quick_sort(more)

print(‘\tLess:’,less,‘\tPivot:’,array[pivot],\

‘\tMore:’,more)array[:]=less+[array[pivot]]+moreprint(‘\t\t…Merged:’,array)

quick.py

Next,addthealgorithmsequencetodividethearraylistelementsintopartitionsthenmergethemtogether

forelementinrange(len(array)):

value=array[element]ifelement!=pivot:

ifvalue<array[pivot]:less.append(value)

else:more.append(value)

Now,addstatementstocreateanddisplaythearraylist

array=[5,3,1,2,6,4]

print(‘QuickSort…\nArray:’,array)quick_sort(array)

print(‘Array:’,array)

Thequicksortalgorithmuseslessmemorythanmergesortandisoftenconsideredtobethebestsortingalgorithm.

Summary• Analgorithmisawell-definedsequenceofinstructionstoperformaspecifictask

• Eachalgorithmtakesoneormorevaluesasinputandproducesoneormoreresultingvaluesasoutput

• Analgorithmmaybecreatedincodeasafunctionwhosestatementsdefineasequenceofinstructionstoperformatask

• Theefficiencyofsortingalgorithmsmaydependuponthenatureofthelisttobesorted

• Asortingalgorithmfunctioncanmakeacopyofalistpassedasinputandreturnasortedcopyofthatlistasoutput

• Thereareseveralpopularalgorithmstosortarraylistsinplacebuteachhavetheirownstrengthsandweaknesses

• SelectionSortalgorithmsrepeatedlyselectthelowestelementintheunsortedsectionofanarraylistandmoveittotheendofthesortedsectionofthatarray

• InsertionSortalgorithmsrepeatedlytakethenextelementintheunsortedsectionofanarraylistandinsertitintothesortedsectionofthatarrayatthecorrectposition

• BubbleSortalgorithmsrepeatedlycompareadjacentelementsinanunsortedarraylistandswapthemintothecorrectorder

• SelectionSort,InsertionSort,andBubbleSortaresimplecomparisonalgorithmsbutarerelativelyslow

• MergeSortalgorithmsrepeatedlydivideanunsortedarraylistintoleftandrightsub-sectionsfromthearray’smid-pointthenmergethesub-sectionsintoasinglesortedlist

• QuickSortalgorithmsrepeatedlydivideanunsortedarraylistintopartitionscontainingvaluesgreaterorlessthanapivotvaluethenmergethepartitionsintoasinglesortedlist

• MergeSortandQuickSortalgorithmsarefastcomplexalgorithmsthateachemployadivide-and-conquerstrategy

8Importinglibraries

Thischapterdemonstrateshowtocodepre-definedlibraryfunctionalityintoyourprograms.

InspectingPythonDoingmathematicsCalculatingdecimalsTellingtimeRunningtimersSummary

InspectingPythonPythonincludes“sys”and“keyword”modulesthatareusefulforinterrogatingthePythonsystemitself.ThekeywordmodulecontainsalistofallPythonkeywordsinitskwlistattributeandprovidesaniskeyword()functionifyouwanttotestanyword.

Youcanexplorethemanyfeaturesofthe“sys”moduleandindeedanyfeatureofPythonusingtheInteractiveModehelpsystem.Justtypehelp()atthe>>>prompttostarttheHelpsystem,thentypesysatthehelp>promptthatappears.

Perhaps,mostusefully,the“sys”modulehasattributesthatcontainthePythonversionnumber,interpreterlocationonyoursystem,andalistofalldirectorieswheretheinterpreterseeksmodulefiles

–soifyousavemodulefilesinanyofthesedirectoriesyoucanbesuretheinterpreterwillfindthem.

system.py

Startanewprogrambyimportingthe“sys”and“keyword”modulestomaketheirfeaturesavailableimportsys,keywordsystem.py

Next,addastatementtodisplaythePythonversion

print(‘PythonVersion:‘,sys.version)

Now,addastatementtodisplaytheactuallocationonyoursystemofthePythoninterpreterprint(‘PythonInterpreterLocation:‘,sys.executable)

Then,addstatementstodisplayalistofalldirectorieswherethePythoninterpreterlooksformodulefilesprint(‘PythonModuleSearchPath:‘)

forfolderinsys.path:print(folder)

Finally,addstatementstodisplayalistallthePythonkeywords

print(‘PythonKeywords:‘)

forwordinkeyword.kwlist:print(word)

Saveandthenruntheprogram–toseedetailsofthePythonversiononyourownsystem

ThefirstitemonthePythonsearchpathisyourcurrentdirectory–soanyfilewithinthereorwithinanysubdirectoriesyoumaketherewillbefoundbythePythoninterpreter.

SpendalittletimewiththeInteractiveModehelputilitytodiscoverlotsmoreaboutPython.

DoingmathematicsPythonincludesa“math”modulethatprovideslotsoffunctionsyoucanusetoperformmathematicalproceduresonceimported.

Themath.ceil()andmath.floor()functionsenableaprogramtoperformroundingofafloatingpointvaluespecifiedbetweentheirparenthesestotheclosestinteger–math.ceil()roundsupandmath.floor()roundsdownbutthevaluereturned,althoughaninteger,isafloatdatatyperatherthananintdatatype.

Themath.pow()functionrequirestwoargumentstoraiseaspecifiedvaluebyaspecifiedpower.Themath.sqrt()function,ontheotherhand,simplyrequiresasingleargumentandreturnsthesquarerootofthatspecifiedvalue.Bothfunctionresultsarereturnedasanumericvalueofthefloatdatatype.

Integerscanbecastfromtheintdatatypetothefloatdatatypeusingthefloat()functionandtothestringdatatypeusingthestr()function.Typicaltrigonometrycanbeperformedusingfunctionsfromthemathmoduletoo,suchasmath.sin(),math.cosin()andmath.tan().

Additionally,Pythonincludesa“random”modulethatcanbeusedtoproducepseudorandomnumbersonceimportedintoaprogram.

Therandom.random()functionproducesasinglefloating-pointnumberbetweenzeroand1.0.Perhaps,moreinterestingly,therandom.sample()functionproducesalistofelementsselectedatrandomfromasequence.Thismethodrequirestwoargumentstospecifythesequencetoselectfrom,andthelengthofthelisttobeproduced.Astherange()functionreturnsasequenceofnumbers,thiscanbeusedtospecifyasequenceasthefirstargumenttotherandom.sample()function–soitwillrandomlyselectnumbersfromthatsequencetoproducealistinwhichnonumbersrepeat.

maths.py

Startanewprogrambyimportingthe“math”and“random”modulestomaketheirfeaturesavailableimportmath,random

Next,addstatementstodisplaytworoundedvalues

print(‘RoundedUp9.5:‘,math.ceil(9.5))

print(‘RoundedDown9.5:‘,math.floor(9.5))

Now,addastatementtoinitializeavariablewithanintegervalue

num=4

Addstatementstodisplaythesquareandsquarerootofthevariablevalue

print(num,‘Squared:‘,math.pow(num,2))

print(num,‘SquareRoot:‘,math.sqrt(num))

Then,addastatementtoproducearandomlistofsixuniquenumbersbetweenoneand49nums=random.sample(range(1,49),6)

Finally,addastatementtodisplaytherandomlist

print(‘YourLuckyLottoNumbersAre:‘,nums)

Savethenruntheprogram–toseemathresultsandrandomsamples

Allthemathfunctionsherereturnfloating-pointnumbersofthefloatdatatype.

Thelistproducedbyrandom.sample()doesnotactuallyreplaceelementsofthesequencebutmerelycopiesasample,asitsnamesays.

CalculatingdecimalsPythonprogramsthatattemptfloating-pointarithmeticcanproduceunexpectedandinaccurateresultsbecausethefloating-pointnumberscannotaccuratelyrepresentalldecimalnumbers.

inaccurate.py

Startanewprogrambyinitializingtwovariableswithfloating-pointvalues

item=0.70

rate=1.05

Next,initializetwomorevariablesbyattemptingfloating-pointarithmeticwiththefirsttwovariablestax=item*ratetotal=item+tax

Now,addstatementstodisplayvariablevaluesformattedtohavetwodecimalplacessotrailingzerosareshownprint(‘Item:\t’,‘%.2f’%item)print(‘Tax:\t’,‘%.2f’%tax)

print(‘Total:\t’,‘%.2f’%total)

Savethenruntheprogram–toseetheoutputdisplayaninaccurateadditiontotal

expanded.py

Tohelpunderstandthisproblemeditallthreeprintstatementstodisplaythevariablevaluesexpandedto20decimalplaces,thenrunthemodifiedprogramprint(‘Item:\t’,‘%.20f’%item)print(‘Tax:\t’,‘%.20f’%tax)

print(‘Total:\t’,‘%.20f’%total)

Here,thevariablevaluesareformattedusingastringsubstitutiontechniquetoshowtwodecimalplaces–describedinmoredetailhere.

ThisproblemisnotuniquetoPython–JavahasaBigDecimalclassthatovercomesthisprobleminmuchthesamewayasthedecimalmoduleinPython.

Itisnowclearthatthetaxvalueisrepresentednumericallyslightlybelow0.735sogetsroundeddownto0.73.Conversely,thetotalvalueisrepresentednumericallyslightlyabove1.435sogetsroundedupto1.44,creatingtheapparentadditionerror.

Errorsinfloating-pointarithmeticcanbeavoidedusingPython’s“decimal”module.ThisprovidesaDecimal()objectwithwhichfloating-pointnumberscanbemoreaccuratelyrepresented.

decimals.py

Addastatementatthebeginningoftheprogramtoimportthe“decimal”moduletomakeallfeaturesavailablefromdecimalimport*

Next,editthefirsttwovariableassignmenttoobjects

item=Decimal(0.70)

rate=Decimal(1.05)

Savethechangesthenrunthemodifiedprogramtoseebothtaxandtotalrepresentationswillnowgetroundeddown–sotheoutputwillshowaccurateadditionwhenstringformattingischangedbacktotwodecimalplaces

AlwaysusetheDecimal()objecttocalculatemonetaryvaluesoranywherethataccuracyisessential.

TellingtimeThePython“datetime”modulecanbeimportedintoaprogramtomakeuseoftimesanddates.Itprovidesadatetimeobjectwithattributesofyear,month,day,hour,minute,second,microsecond.

Adatetimeobjecthasatoday()functionthatassignsthecurrentdateandtimevaluestoitsattributesandreturnstheminatuple.Italsohasagetattr()functionthatrequirestwoargumentsspecifyingthedatetimeobjectnameandattributetoretrieve.Alternatively,theattributescanbereferencedusingdotnotationsuchasdatetime.year.

Asthedatetimeobjectisinamoduleofthesamename,simplyimportingthemodulemeansitwouldbereferencedasdatetime.datetime.Usefromdatetimeimport*soitcanbereferencedjustasdatetimealone.Allvaluesinadatetimeobjectarestoredasnumericvaluesbutcanbeusefullytransformedintotextequivalentsusingitsstrftime()function.Thisrequiresasinglestringargumentthatisa“directive”specifyingwhichpartofthetupletoreturnandinwhatformat.Thepossibledirectivesarelistedinthetablebelow:

Directive: Returns:

%A Fullweekdayname(%aforabbreviateddayname)

%B Fullmonthname(%bforabbreviatedmonthname)

%c Dateandtimeappropriateforlocale

%d Dayofthemonthnumber1-31

%f Microsecondnumber0-999999

%H Hournumber0-23(24-hourclock)

%I Hournumber1-12(12-hourclock)

%j Dayoftheyearnumber0-366

%m Monthnumber1-12

%M Minutenumber0-59

%p AMorPMequivalentforlocale

%S Secondnumber0-59

%w Weekdaynumber0(Sunday)-6

%W Weekoftheyearnumber0-53

%X Timeappropriateforlocale(%xforappropriatedate)

%Y Year0001-9999(%yforyear00-99)

%z TimezoneoffsetfromUTCas+HHMMor-HHMM

%Z Timezonename

Asthestrftime()functionrequiresastringargument,thedirectivemustbeenclosedbetweenquotemarks.

time.py

Startanewprogrambyimportingthe“datetime”moduletomakeitsfeaturesavailablefromdatetimeimport*

Next,createadatetimeobjectwithattributesassignedcurrentdateandtimevaluesthendisplayitscontentstoday=datetime.today()

print(‘TodayIs:‘,today)

Addalooptodisplayeachattributevalueindividually

forattrin\[‘year’,‘month’,‘day’,‘hour’,‘minute’,’second’,’microsecond’]:

print(attr,‘:\t’,getattr(today,attr))

Now,addastatementtodisplaytimeusingdotnotation

print(‘Time:‘,today.hour,‘:’,today.minute,sep=‘‘)

Then,assignformatteddayandmonthnamestovariables

day=today.strftime(‘%A’)

month=today.strftime(‘%B’)

Finally,addastatementtodisplaytheformatteddate

print(‘Date:‘,day,month,today.day)

Savethenruntheprogram–toseethedateandtimevaluesgetdisplayed

Noticehowthe\backslashcharacterisusedinthislooptoallowastatementtocontinueonthenextlinewithoutcausinganerror.

Youcanassignnewvaluestoattributesofadatetimeobjectusingitsreplace()function,suchastoday=today.replace(year=2015)

RunningtimersGettingthecurrenttimebothbeforeandafteraneventmeansthatthedurationoftheeventcanbecalculatedbytheirdifference.ThePython“time”modulecanbeimportedintoaprogramtoprovidevarioustime-relatedfunctions.

CurrentsystemtimeisusuallycountedasthenumberofsecondselapsedsincetheEpochat00:00:00GMTonJanuary1,1970.Thetimemodule’stime()functionreturnsthecurrenttimeinsecondssincetheEpochasafloatingpointnumberwhencalled.

Thegmtime()functionconvertselapsedtimefromtheEpochtoastruct_timeobjectatUTCwiththeDaylightSavingTimealwayssettozero,whereaslocaltime()convertstoastruct_timeobjectatyourlocalsystemtime.Thefigurereturnedbythetime()functioncanbeconvertedintoa“struct_time”objectusinggmtime()orlocaltime()functions.Thisobjecthasattributesoftm_year,tm_mon,tm_mday,tm_hour,tm_min,tm_sec,tm_wday,tm_yday,tm_ydayandtm_isdstthatcanbereferencedusingdotnotation.Forexample,struct.tm_wday.

Allvaluesinastruct_timeobjectarestoredasnumericvaluesbutcanbetransformedintotextequivalentsusingthestrftime()function.Thisrequiresanargumentthatisaformat“directive”followedbythenameofthestruct_timeobject.Thepossibledirectivesincludethoselistedinthetablehereforthedatetimeobject.Forexample,strftime(‘%A’,struct)forweekday.

Usefully,thetimemodulealsoprovidesasleep()functionthatcanbeusedtopauseexecutionofaprogram.Itsargumentspecifiestheamountoftimeinsecondsbywhichtodelayexecution.

timer.py

Startanewprogrambyimportingthe“time”moduletomakeitsfeaturesavailable

fromtimeimport*

Next,initializeavariablewithafloatingpointnumberthatisthecurrentelapsedtimesincetheepochstart_timer=time()

Now,addastatementtocreateastruct_timeobjectfromtheelapsedtimevalue

struct=localtime(start_timer)

Then,announcethatacountdowntimerisabouttobeginfromthecurrenttimestartingpointprint(‘StartingCountdownAt:’,strftime(‘%X’,struct))

Addalooptoinitializeandprintacountervariablevaluethendecrementthecounterbyoneandpauseforonesecondoneachiterationi=10

whilei>-1:print(i)

i-=1sleep(1)

Next,initializeavariablewithafloatingpointnumberthatisthecurrentelapsedtimenowsincetheEpochend_timer=time()

Now,initializeavariablewiththeroundedsecondsvalueofthetimedifferencebetweenthetwotimedpointsdifference=round(end_timer-start_timer)

Finally,addastatementtodisplaythetimetakentoexecutethecountdownloop

print(‘\nRuntime:’,difference,‘Seconds’)

Savethenruntheprogram–toseethelooppauseoneachiterationandelapsedtime

Theargumenttothesleep()functionmaybeafloatingpointnumbertoindicateamoreprecisesleeppausetime.

Donotconfusethetime.strftime()functionusedinthisexamplewiththedatetime.strftime()functionusedinthepreviousexample.

Summary• ThesysmodulehasattributesthatcontainthePythonversionnumber,interpreter

location,andpathtosearchformodules

• ThekeywordmodulehasakwlistattributethatcontainsalistofallcurrentPythonkeywords

• Themathmoduleprovidesfunctionstoperformmathematicalproceduressuchasmath.ceil()andmath.floor()

• Themath.pow()andmath.sqrt()functionsbothreturntheirresultsasadecimalvalueofthefloatdatatype

• Trigonometrycanbeperformedusingmathmodulefunctionssuchasmath.sin(),math.cosin()andmath.tan()

• Therandommoduleprovidesarandom()functionthatproducespseudorandomnumbersandasample()functionthatproducesalistofelementsselectedatrandomfromasequence

• Floating-pointfloatnumberscannotaccuratelyrepresentalldecimalnumbers

• ThedecimalmoduleprovidesaDecimal()objectwithwhichfloating-pointnumberscanbeaccuratelyrepresentedtocalculatemonetaryvalues

• Thedatetimemoduleprovidesadatetimeobjectwithyear,month,day,hour,minute,second,microsecondattributesthatcanbereferencedbydot-suffixingorwiththegetattr()function

• Adatetimeobjecthasastrftime()functionthatcanspecifyadirectivetoreturnaformattedpartoftheobject

• Thetimemoduleprovidesatime()functionthatreturnsthecurrentelapsedtimeinsecondssincetheEpoch

• Thegmtime()andlocaltime()functionsreturnastruct_timeobjectwithattributescontainingdateandtimecomponents

9Managingtext

Thischapterdemonstrateshowtomanipulatetextstringsinyourprogramsandhowtostoretextinfiles.

ManipulatingstringsFormattingstringsModifyingstringsAccessingfilesManipulatingcontentUpdatingcontentSummary

ManipulatingstringsStringvaluescanbemanipulatedinaPythonprogramusingthevariousoperatorslistedinthetablebelow:

Operator: Description: Example:

+ Concatenate–joinstringstogether ‘Hello’+‘Mike’

* Repeat–multiplythestring ‘Hello’*2

[] Slice–selectacharacterataspecifiedindexposition ‘Hello’[0]

[:] RangeSlice–selectcharactersinaspecifiedindexrange ‘Hello’[0:4]

in MembershipInclusive–returnTrueifcharacterexistsinthestring ‘H’in‘Hello’

notin MembershipExclusive–returnTrueifcharacterdoesn’texistinstring ‘h’notin‘Hello’

r/R RawString–suppressmeaningofescapecharacters print(r’\n’)

‘‘‘‘‘‘ Docstring–describeamodule,function,class,ormethod defsum(a,b):‘‘‘AddArgs‘‘‘

Themembershipoperatorsperformacase-sensitivematch,so‘A’in‘abc’willfail.

The[]sliceoperatorand[:]rangesliceoperatorrecognizethatastringissimplyalistcontaininganindividualcharacterwithineachlistelement,whichcanbereferencedbytheirindexnumber.

Similarly,theinandnotinmembershipoperatorsiteratethrougheachelementseekingtomatchthespecifiedcharacter.

Therawstringoperatorr(oruppercaseR)mustbeplacedimmediatelybeforetheopeningquotemarktosuppressescapecharactersinthestringandisusefulwhenthestringcontainsthebackslashcharacter.

TheRangeSlicereturnsthestringupto,butnotincluding,thefinalspecifiedindexposition.

A“docstring”isadescriptivestringliteralthatoccursasthefirststatementinamodule,afunction,aclass,oramethoddefinition.Thisshouldbeenclosedwithintriplesinglequotemarks.Uniquely,thedocstringbecomesthe__doc__specialattributeofthatobject,socanbereferencedusingitsnameanddot-suffixing.Allmodulesshouldnormallyhavedocstrings,andallfunctionsandclassesexportedbyamoduleshouldalsohavedocstrings.

manipulate.py

Startanewprogrambydefiningasimplefunctionthatincludesadocstringdescriptiondefdisplay(s):

‘‘’Displayanargumentvalue.’’’print(s)

Next,addastatementtodisplaythefunctiondescription

display(display.doc)

Now,addastatementtodisplayarawstringvaluethatcontainsthebackslashcharacterdisplay(r’C:\ProgramFiles’)

Then,addastatementtodisplayaconcatenationoftwostringvaluesthatincludeanescapecharacterandaspacedisplay(‘\nHello’+‘Python’)

Next,addastatementtodisplayasliceofaspecifiedstringwithinarangeofelementindexnumbersdisplay(‘PythonInEasySteps\n’[7:])

Finally,displaytheresultsofseekingcharacterswithinaspecifiedstring

display(‘P’in‘Python’)

display(‘p’in‘Python’)

Savethenruntheprogram–toseemanipulatedstringsgetdisplayed

Rememberthatstringsmustbeenclosedwithineithersinglequotemarksordoublequotemarks.

Withrangeslice,ifthestartindexnumberisomittedzeroisassumedandiftheendindexnumberisomittedthestringlengthisassumed.

FormattingstringsThePythonbuilt-indir()functioncanbeusefultoexaminethenamesoffunctionsandvariablesdefinedinamodulebyspecifyingthemodulenamewithinitsparentheses.Interactivemodecaneasilybeusedforthispurposebyimportingthemodulenamethencallingthedir()function.Theexamplebelowexaminesthe“dog”modulecreatedhereinChapterSix:

Noticethatthe__doc__attributeintroducedinthepreviousexampleappearslistedherebythedir()function.ThosedefinednamesthatbeginandendwithadoubleunderscorearePythonobjects,whereastheothersareprogrammer-defined.The__builtins__modulecanalsobeexaminedusingthedir()function,torevealthenamesoffunctionsandvariablesdefinedbydefault,suchastheprint()functionandastrobject.

Thestrobjectdefinesseveralusefulfunctionsforstringformatting,includinganactualformat()functionthatperformsreplacements.Astringtobeformattedbytheformat()functioncancontainbothtextand“replacementfields”markingplaceswheretextistobeinsertedfromanorderedcomma-separatedlistofvalues.Eachreplacementfieldisdenotedby{}braces,whichmayoptionallycontaintheindexnumberpositionofthereplacementinthelist.

Donotconfusethestrobjectdescribedherewiththestr()functionthatconvertsvaluestothestringdatatype.

StringsmayalsobeformattedusingtheC-style%ssubstitutionoperatortomarkplacesinastringwheretextistobeinsertedfromacomma-separatedorderedlistofvalues.

format.py

Startanewprogrambyinitializingavariablewithaformattedstring

snack=‘{}and{}’.format(‘Burger’,‘Fries’)

Next,displaythevariablevaluetoseethetextreplacedintheirlistedorder

print(‘\nReplaced:’,snack)

Now,assignadifferentlyformattedstringtothevariable

snack=‘{1}and{0}’.format(‘Burger’,‘Fries’)

Then,displaythevariablevalueagaintoseethetextnowreplacedbytheirspecifiedindexelementvalueprint(‘Replaced:’,snack)

Assignanotherformattedstringtothevariable

snack=‘%sand%s’%(‘Milk’,‘Cookies’)

Finally,displaythevariablevalueoncemoretoseethetextsubstitutedintheirlistedorderprint(‘\nSubstituted:’,snack)

Savethenruntheprogram–toseeformattedstringsgetdisplayed

Youcannotleavespacesaroundtheindexnumberinthereplacementfield.

Otherdatatypescanbesubstitutedusing%dforadecimalinteger,%cforacharacter,and%fforafloating-pointnumber.

ModifyingstringsThePythonstrobjecthasmanyusefulfunctionsthatcanbedot-suffixedtoitsnameformodificationofthestringandtoexamineitscontents.Mostcommonlyusedstringmodificationfunctionsarelistedinthetablebelowtogetherwithabriefdescription:

Method: Description:

capitalize() Changestring’sfirstlettertouppercase

title() Changeallfirstletterstouppercase

upper()lower()swapcase()

Changethecaseofallletterstouppercase,tolowercase,ortotheinverseofthecurrentcaserespectively

join(seq) Mergestringintoseparatorsequenceseq

lstrip()rstrip()strip()

Removeleadingwhitespace,trailingwhitespace,orbothleadingandtrailingwhitespacerespectively

replace(old,new) Replacealloccurrencesofoldwithnew

ljust(w,c)rjust(w,c) Padstringtorightorleftrespectivelytototalcolumnwidthwwithcharacterc

center(w,c) Padstringeachsidetototalcolumnwidthwwithcharacterc(defaultisspace)

count(sub) Returnthenumberofoccurrencesofsub

find(sub) Returntheindexnumberofthefirstoccurrenceofsuborreturn-1ifnotfound

startswith(sub)endswith(sub) ReturnTrueifsubisfoundatstartorendrespectively–otherwisereturnFalse

isalpha()isnumeric()isalnum()

ReturnTrueifallcharactersarelettersonly,arenumbersonly,arelettersornumbersonly–otherwisereturnFalse

islower()isupper()istitle()

ReturnTrueifstringcharactersarelowercase,uppercase,orallfirstlettersareuppercaseonly–otherwisereturnFalse

isspace() ReturnTrueifstringcontainsonlywhitespace–otherwisereturnFalse

isdigit()isdecimal() ReturnTrueifstringcontainsonlydigitsordecimals–otherwisereturnFalse

Aspacecharacterisnotalphanumericsoisalnum()returnsFalsewhenexaminingstringsthatcontainspaces.

modify.py

Startanewprogrambyinitializingavariablewithastringoflowercasecharactersandspacesstring=‘codingforbeginnersineasysteps’

Next,displaythestringcapitalized,titled,andcentered

print(‘\nCapitalized:\t’,string.capitalize())

print(‘\nTitled:\t\t’,string.title())print(‘\nCentered:\t’,string.center(30,‘*’))

Now,displaythestringinalluppercaseandmergedwithasequenceoftwoasterisksprint(‘\nUppercase:\t’,string.upper())

print(‘\nJoined:\t\t’,string.join(‘**’))

Then,displaythestringpaddedwithasterisksontheleft

print(‘\nJustified:\t\t’,string.rjust(30,‘*’))

Finally,displaythestringwithalloccurrencesofthe‘s’

characterreplacedbyasterisksprint(‘\nReplaced:\t’,string.replace(‘s’,‘*’))

Savethenruntheprogram–toseemodifiedstringsgetdisplayed

Withtherjust()functionaRIGHT-justifiedstringgetspaddingaddedtoitsLEFT,andwiththeljust()functionaLEFT-justifiedstringgetspaddingaddedtoitsRIGHT.

AccessingfilesThe__builtins__modulecanbeexaminedusingthedir()functiontorevealthatitcontainsafileobjectthatdefinesseveralmethodsforworkingwithfiles,includingopen(),read(),write(),andclose().

Beforeafilecanbereadorwrittenitfirstlymustalwaysbeopenedusingtheopen()function.Thisrequirestwostringargumentstospecifythenameandlocationofthefile,andoneofthefollowing“mode”specifiersinwhichtoopenthefile:

Filemode: Operation:

r Openanexistingfiletoread

w Openanexistingfiletowrite.Createsanewfileifnoneexistsoropensanexistingfileanddiscardsallitspreviouscontents

a Appendtext.Opensorcreatesatextfileforwritingattheendofthefile

r+ Openatextfiletoreadfromorwriteto

w+ Openatextfiletowritetoorreadfrom

a+ Openorcreatesatextfiletoreadfromorwritetoattheendofthefile

Wherethemodeincludesabafteranyofthefilemodeslistedabove,theoperationrelatestoabinaryfileratherthanatextfile.Forexample,rborw+b

Filemodeparametersarestringvaluessomustbesurroundedbyquotes.

Onceafileisopenedandyouhaveafileobjectandcangetvariousinformationrelatedtothatfilefromitsproperties:

Property: Description:

name Nameoftheopenedfile

mode Modeinwhichthefilewasopened

closed StatusBooleanvalueofTrueorFalse

readable() ReadpermissionBooleanvalueofTrueorFalse

writable() WritepermissionBooleanvalueofTrueorFalse

Youcanalsouseareadlines()functionthatreturnsalistofalllines.

access.py

Startanewprogrambycreatingafileobjectforanewtextfilenamed“example.txt”inwhichtowritecontentfile=open(‘example.txt’,‘w’)

Next,addstatementstodisplaythefilenameandmode

print(‘FileName:’,file.name)

print(‘FileOpenMode:’,file.mode)

Now,addstatementstodisplaythefileaccesspermissions

print(‘Readable:’,file.readable())print(‘Writable:’,file.writable())

Then,defineafunctiontodeterminethefile’sstatus

defget_status(f):

if(f.closed!=False):return‘Closed’

else:return‘Open’

Finally,addstatementstodisplaythecurrentfilestatusthenclosethefileanddisplaythefilestatusoncemoreprint(‘FileStatus:’,get_status(file))

file.close()print(‘\nFileStatus:’,get_status(file))

Savethenruntheprogram–toseeafilegetopenedforwriting,thenseethefilegetclosed

Ifyourprogramtriestoopenanon-existentfileinrmodetheinterpreterwillreportanerror.

ManipulatingcontentOnceafilehasbeensuccessfullyopeneditcanberead,oraddedto,ornewtextcanbewritteninthefile,dependingonthemodespecifiedinthecalltotheopen()function.Followingthis,theopenfilemustalwaysbeclosedbycallingtheclose()function.

Asyoumightexpect,theread()functionreturnstheentirecontentofthefileandthewrite()functionaddscontenttothefile.

Youcanquicklyandefficientlyreadtheentirecontentsinaloop,iteratinglinebyline.

readwrite.py

Startanewprogrambyinitializingavariablewithaconcatenatedstringcontainingnewlinecharacterspoem=‘Ineversawamanwholooked\n’

poem+=‘Withsuchawistfuleye\n’poem+=‘Uponthatlittletentofblue\n’

poem+=‘Whichprisonerscallthesky\n’

Next,addastatementtocreateafileobjectforanewtextfilenamed“poem.txt”towritecontentintofile=open(‘poem.txt’,‘w’)

Now,addstatementstowritethestringcontainedinthevariableintothetextfile,thenclosethatfilefile.write(poem)file.close()

Then,addastatementtocreateafileobjectfortheexistingtextfile“poem.txt”toreadfromfile=open(‘poem.txt’,‘r’)

Now,addstatementstodisplaythecontentsofthetextfile,thenclosethatfile

forlineinfile:

print(line,end=‘’)file.close()

Savethenruntheprogram–toseethefilecreatedandreadouttodisplay

LaunchtheNotepadtexteditortoconfirmthenewtextfileexistsandrevealitscontentswrittenbytheprogram

Now,addstatementsattheendoftheprogramtoappendacitationtothetextfilethensavethescriptfileagainfile=open(‘poem.txt’,‘a’)

file.write(‘(OscarWilde)’)file.close()

Savethenruntheprogramagaintore-writethetextfilethenviewitscontentsinNotepad–toseethecitationnowappendedaftertheoriginaltextcontent

Writingtoanexistingfilewillautomaticallyoverwriteitscontents!

Suppressthedefaultnewlineprovidedbytheprint()functionwherethestringsthemselvescontainnewlines.

Youcanalsousethefileobject’sreadlines()functionthatreturnsalistofalllinesinafile–onelineperelement.

UpdatingcontentAfileobject’sread()functionwill,bydefault,readtheentirecontentsofthefilefromtheverybeginning,atindexpositionzero,totheveryend–attheindexpositionofthefinalcharacter.Optionally,theread()functioncanacceptanintegerargumenttospecifyhowmanycharactersitshouldread.

Thepositionwithinthefile,fromwhichtoreadoratwhichtowrite,canbefinelycontrolledbythefileobject’sseek()function.Thisacceptsanintegerargumentspecifyinghowmanycharacterstomovepositionasanoffsetfromthestartofthefile.

Thecurrentpositionwithinafilecanbediscoveredatanytimebycallingthefileobject’stell()functiontoreturnanintegerlocation.

WhenworkingwithfileobjectsitisgoodpracticetousethePythonwithkeywordtogroupthefileoperationalstatementswithinablock.Thistechniqueensuresthatthefileisproperlyclosedafteroperationsend,evenifanexceptionisraisedontheway,andmuchshorterthanwritingequivalenttryexceptblocks.

update.py

Startanewprogrambyassigningastringvaluetoavariablecontainingtexttobewritteninafiletext=‘Thepoliticalslogan“WorkersOfTheWorldUnite!”\nisfromTheCommunistManifesto.’

Next,addstatementstowritethetextstringintoafileanddisplaythefile’scurrentstatusinthe“with”blockwithopen(‘update.txt’,‘w’)asfile:

file.write(text)print(‘\nFileNowClosed?:’,file.closed)

Now,addanon-indentedstatementafterthe“with”codeblocktodisplaythefile’snewstatusprint(‘FileNowClosed?:’,file.closed)

Then,re-openthefileanddisplayitscontentstoconfirmitnowcontainstheentiretextstringwithopen(‘update.txt’,‘r+’)asfile:

text=file.read()print(‘\nString:’,text)

Next,addindentedstatementstodisplaythecurrentfileposition,thenreposition

anddisplaythatnewpositionprint(‘\nPositionInFileNow:’,file.tell())

position=file.seek(33)print(‘PositionInFileNow:’,file.tell())

Now,addanindentedstatementtooverwritethetextfromthecurrentfileposition

file.write(‘AllLands’)

Then,addindentedstatementstorepositioninthefileoncemoreandoverwritethetextfromthenewposition

file.seek(61)

file.write(‘thetombstoneofKarlMarx.’)

Finally,addindentedstatementstoreturntothestartofthefileanddisplayitsentireupdatedcontents

file.seek(0)text=file.read()

print(‘\nString:’,text)

Savethenruntheprogram–toseethefilestringsgetupdated

Theseek()functionmayoptionallyacceptasecondargumentvalueof0,1,or2tomovethespecifiednumberofcharactersfromthestart,current,orendpositionrespectively–zeroisthedefaultstartposition.

Aswithstrings,thefirstcharacterinafileisatindexpositionzero–notatindexpositionone.

Summary• Stringscanbemanipulatedbyoperatorsforconcatenation+,tojoinstringstogether,

andforrepetition*ofstrings

• Stringscanbemanipulatedbyoperatorsforslice[],andrangeslice[:],thatreferencetheindexnumberofstringcharacters

• Stringscanbemanipulatedbymembershipoperatorsinandnotinthatseektomatchaspecifiedcharacterwithinastring

• Ther(orR)rawstringoperatorcanbeplacedimmediatelybeforeastringtosuppressanyescapecharactersitcontains

• A“docstring”isadescriptivestringwithintriplequotemarksatthestartofamodule,class,orfunction,todefineitspurpose

• The__doc__attributecanbeusedtoreferencethestringdescriptionwithinadocstring

• The__builtins__modulecanbeexaminedusingthedir()functiontorevealthenamesofdefaultfunctionsandvariables

• Astrobjecthasaformat()functionforstringformattingandmanyfunctionsforstringmodification,suchascapitalize()

• Replacementfieldsinastringtobeformattedusingtheformatfunctionaredenotedinacomma-separatedlistby{}braces

• StringscanalsobeformattedusingtheC-style%ssubstitutionoperatortomarkplacesinastringwheretextistobeinserted

• Afileobjecthasopen(),read(),write(),andclose()functionsforworkingwithfiles,andfeaturesthatdescribethefileproperties

• Theopen()functionmustspecifyafilenamestringargumentandafilemodestringparameter,suchas’r’toreadthefile

• Aopenedfileobjecthasinformationpropertiesthatrevealitscurrentstatus,suchasmodeandreadable()values

• Positioninafile,atwhichtoreadorwrite,canbespecifiedwiththeseek()methodandreportedbythetell()function

• ThePythonwithkeywordgroupsfileoperationalstatementswithinablockandautomaticallyclosesanopenfile

10Programmingobjects

Thischapterdemonstrateshowtocodevirtualobjectsintoyourprograms.

DefiningclassesCopyinginstancesAddressingpropertiesDerivingclassesOverridingmethodsApplyingsenseSummary

DefiningclassesThereal-worldobjectsthatsurrounduseachhaveattributesandbehaviorswecandescribe.Forexample,acarmightbedescribedwithacolorattribute“red”andan“acceleration”behavior.Programmingobjectsarelikevirtualrepresentationsofreal-worldobjectsthatdescribeattributesandbehaviorsina“class”structure.

A“class”isaspecifiedprototypedescribingasetofpropertiesthatcharacterizeanobject.Eachclasshasadatastructurethatcancontainbothfunctionsandvariablestocharacterizetheobject.Thepropertiesofaclassarereferredtoasitsdata“members”.Classfunctionmembersareknownasits“methods”,andclassvariablemembers(declaredwithinaclassstructurebutoutsideanymethoddefinitions)areknownasits“attributes”.Classmemberscanbereferencedthroughoutaprogramusingdotnotation,suffixingthemembernameaftertheclassname,withsyntaxofclass-name.method-name()orclass-name.attribute-name.

Aclassdeclarationbeginswiththeclasskeyword,followedbyaprogrammer-specifiedname(adheringtotheusualPythonnamingconventionsbutbeginninginuppercase)thena:colon.Nextcomeindentedstatements,optionallyspecifyingaclassdocumentstring,classvariableattributedeclarations,andclassmethoddefinitions–sotheclassblocksyntaxlookslikethis:classClassName:

‘‘‘class-documentation-string‘‘‘class-variable-declarations

class-method-definitions

Theclassdeclaration,whichspecifiesitsattributesandmethods,isablueprintfromwhichworkingcopies(“instances”)canbemade.Allvariablesdeclaredwithinmethoddefinitionsareknownas“instance”variablesandareonlyavailablelocallywithinthemethodinwhichtheyaredeclared–theycannotbedirectlyreferencedoutsidetheclassstructure.

Itisconventionaltobeginclassnameswithanuppercasecharacterandobjectnameswithlowercase.

Typically,instancevariablescontaindatapassedbythecallerwhenaninstancecopyoftheclassiscreated.Asthisdataisonlyavailablelocallyforinternaluseitiseffectively

hiddenfromtherestoftheprogram.Thistechniqueofdata“encapsulation”ensuresthatdataissecurelystoredwithintheclassstructureandisthefirstprincipleofObjectOrientedProgramming(OOP).

Allpropertiesofaclassarereferencedinternallybythedotnotationprefixself–soanattributenamed“sound”isself.sound.Additionally,allmethoddefinitionsinaclassmusthaveselfastheirfirstparameter–soamethodnamed“talk”istalk(self).

Whenaclassinstanceiscreated,aspecial__init__(self)methodisautomaticallycalled.Subsequentparameterscanbeaddedinitsparenthesesifvaluesaretobepassedtoinitializeitsattributes.

AcompletePythonclassdeclarationcouldlooklikethisexample:classCritter:

‘‘‘Abaseclassforallcritterproperties.‘‘‘count=0

def__init__(self,chat):self.sound=chat

Critter.count+=1deftalk(self):

returnself.sound

Theclassdocumentationstringcanbeaccessedviathespecial__doc__docstringattributewithClassname.__doc__.

Itisusefultoexaminetheclasscomponentsofthisexample:

• Thevariablecountisaclassvariablewhoseintegervaluegetssharedamongallinstancesofthisclass–thisvaluecanbereferencedasCritter.countfrominsideoroutsidetheclass

• Thefirstmethod__init__()istheinitializationmethodthatisautomaticallycalledwhenaninstanceoftheclassiscreated

• The__init__()methodinthiscaseinitializesaninstancevariablesound,withavaluepassedfromthechatparameter,andincrementsthevalueofthecountclassvariablewheneveraninstanceofthisclassiscreated

• Thesecondmethodtalk()isdeclaredlikearegularfunctionexceptthefirstparameterisselfwhichisautomaticallyincorporated–novalueneedstobepassedfromthecaller

• Thetalk()methodinthiscasesimplyreturnsthevalueencapsulatedinthesoundinstancevariable

Whileaprogramclasscannotperfectlyemulateareal-worldobject,theaimistoencapsulateallrelevantattributesandactions.

CopyinginstancesAn“instance”ofaclassobjectissimplyacopyoftheprototypecreatedbycallingthatclassname’sconstructorandspecifyingtherequirednumberofparameterswithinitsparentheses.Thecall’sargumentsmustmatchthosespecifiedbythe__init__()methoddefinition–otherthanavaluefortheinternalselfparameter.

Theclassinstanceobjectreturnedbytheconstructorisassignedtoavariableusingthesyntaxinstance-name=ClassName(args).

Dotnotationcanbeusedtoreferencethemethodsandclassvariableattributesofaninstanceobjectbysuffixingtheirnameasinstance-name.method-name()orinstance-name.attribute-name.

Aconstructorcreatesaclassinstanceusingtheclassnamefollowedbyparenthesescontaininganyrequiredparameters.

Typically,abaseclasscanbedefinedasaPythonmodulefilesoitcanbeimportedintootherscriptswhereinstanceobjectscanbeeasilycreatedfromthe“master”classprototype.

Bird.py

Startanewclassfilebydeclaringanewclasswithadescriptivedocumentstring

classBird:

‘‘’Abaseclasstodefinebirdproperties.’’’

Next,addanindentedstatementtodeclareandinitializeaclassvariableattributewithanintegerzerovalue

count=0

Now,definetheintializerclassmethodtoinitializeaninstancevariableandtoincrementtheclassvariable

def__init__(self,chat):self.sound=chat

Bird.count+=1

Finally,addaclassmethodtoreturnthevalueoftheinstancevariablewhencalled–thensavethisclassfile

deftalk(self):

returnself.sound

YoumustnotpassanargumentvaluefortheselfparameterasthisisautomaticallyincorporatedbyPython.

instance.py

StartaprogrambymakingfeaturesoftheclassfileavailablethendisplayitsdocumentstringfromBirdimport*print(‘\nClassInstancesOf:\n’,Bird.__doc__)

Next,addastatementtocreateaninstanceoftheclassandpassastringargumentvaluetoitsinstancevariablepolly=Bird(‘Squawk,squawk!’)

Now,displaythisinstancevariablevalueandcalltheclassmethodtodisplaythecommonclassvariablevalueprint(‘\nNumberOfBirds:’,polly.count)

print(‘PollySays:’,polly.talk())

Birdinstance–polly

Createasecondinstanceoftheclasspassingadifferentstringargumentvaluetoitsinstancevariableharry=Bird(‘Tweet,tweet!’)

Finally,displaythisinstancevariablevalueandcalltheclassmethodtodisplaythecommonclassvariablevalueprint(‘\nNumberOfBirds:’,harry.count)print(‘HarrySays:’,harry.talk())

Birdinstance–harry

Savebothfilesthenruntheprogram–toseetwoinstancesoftheBirdclassgetcreated

TheclassvariablecountcanalsobereferencedwithBird.countbuttheencapsulatedinstancevariablesoundcanonlybeaccessedbycallinganinstance’stalk()method.

AddressingpropertiesAnattributeofaclassinstancecanbeadded,modified,orremovedatanytimeusingdotnotationtoaddresstheattribute.Makingastatementthatassignsavaluetoanattributewillupdatethevaluecontainedwithinanexistingattributeorcreateanewattributeofthespecifiednamecontainingtheassignedvalue:instance-name.attribute-name=value

delinstance-name.attribute-name

Alternatively,youcanusethefollowingPythonbuilt-infunctionstoadd,modify,orremoveaninstancevariable:

• getattr(instance-name,‘attribute-name‘)–returntheattributevalueoftheclassinstance

• hasattr(instance-name,‘attribute-name‘)–returnTrueiftheattributevalueexistsintheinstance,otherwisereturnFalse

• setattr(instance-name,‘attribute-name‘,value)–updatetheexistingattributevalueorcreateanewattributeintheinstance

• delattr(instance-name,‘attribute-name‘)–removetheattributefromtheinstance

Theattributenamespecifiedtothesebuilt-infunctionsmustbeenclosedwithinquotes.

ThenameofattributesautomaticallysuppliedbyPythonalwaysbeginwithanunderscorecharactertonotionallyindicate“privacy”–sotheseshouldnotbemodified,orremoved.Youcanaddyourownattributesnamedinthiswaytoindicateprivacyifyouwish,butinrealitythesecanbemodifiedlikeanyotherattribute.

address.py

StartanewprogrambymakingfeaturesoftheBirdclassavailablethatwascreatedherefromBirdimport*

Next,createaninstanceoftheclassthenaddanewattributewithanassignedvalueusingdotnotationchick=Bird(‘Cheep,cheep!’)chick.age=‘1week’

Now,displaythevaluesinbothinstancevariableattributes

print(‘\nChickSays:’,chick.talk())

print(‘ChickAge:’,chick.age)

Then,modifythenewattributeusingdotnotationanddisplayitsnewvalue

chick.age=‘2weeks’print(‘ChickNow:’,chick.age)

Next,modifythenewattributeoncemore,thistimeusingabuilt-infunction

setattr(chick,‘age’,‘3weeks’)

Now,displayalistofallnon-privateinstanceattributesandtheirrespectivevaluesusingabuilt-infunctionprint(‘\nChickAttributes…’)

forattribindir(chick):ifattrib[0]!=‘_’:

print(attrib,‘:’,getattr(chick,attrib))

Finally,removethenewattributeandconfirmitsremovalusingabuilt-infunction

delattr(chick,‘age’)print(‘\nChickageAttribute?’,hasattr(chick,‘age’))

Savethenruntheprogram–toseetheinstanceattributesgetaddressed

Birdinstance–chick

Thisloopskipsanyattributewhosenamebeginswithanunderscore,so“private”attributeswillnotgetdisplayedinthelist.

DerivingclassesAPythonclasscanbecreatedasabrandnewclass,likethoseinpreviousexamples,orcanbe“derived”fromanexistingclass.Importantly,aderivedclassinheritsmembersoftheparent(base)classfromwhichitisderived–inadditiontoitsownmembers.

Theabilitytoinheritmembersfromabaseclassallowsderivedclassestobecreatedthatsharecertaincommonproperties,whichhavebeendefinedinthebaseclass.Forexample,a“Polygon”baseclassmaydefinewidthandheightpropertiesthatarecommontoallpolygons.Classesof“Rectangle”andTriangle”couldbederivedfromthePolygonclass–inheritingwidthandheightproperties,inadditiontotheirownmembersdefiningtheiruniquefeatures.

ThevirtueofinheritanceisextremelypowerfulandisthesecondprincipleofObjectOrientedProgramming(OOP).

Aderivedclassdeclarationadds()parenthesesafteritsclassnamespecifyingthenameofitsparentbaseclass.

Polygon.py

CreateanewclassfilethatdeclaresabaseclasswithtwoclassvariablesandamethodtosettheirvaluesclassPolygon:

width=0height=0

defset_values(self,width,height):Polygon.width=width

Polygon.height=height

Rectangle.py

Next,createaclassfilethatdeclaresaderivedclasswithamethodtoreturnmanipulatedclassvariablevaluesfromPolygonimport*

classRectangle(Polygon):defarea(self):

returnself.width*self.height

Triangle.py

Now,createanotherclassfilethatdeclaresaderivedclasswithamethodtoreturnmanipulatedclassvariablevaluesfromPolygonimport*classTriangle(Polygon):

defarea(self):return(self.width*self.height)/2

SavethethreeclassfilesthenstartanewprogrambymakingfeaturesofbothderivedclassesavailablefromRectangleimport*

fromTriangleimport*

Next,createaninstanceofeachderivedclass

rect=Rectangle()trey=Triangle()

Now,calltheclassmethodinheritedfromthebaseclass,passingargumentstoassigntotheclassvariablesrect.set_values(4,5)

trey.set_values(4,5)

Finally,displaytheresultofmanipulatingtheclassvariablesinheritedfromthebaseclassprint(‘RectangleArea:’,rect.area())print(‘TriangleArea:’,trey.area())

Savethenruntheprogram–toseeoutputgetdisplayedusinginheritedfeatures

inherit.py

Aclassdeclarationcanderivefrommorethanoneclassbylistingmultiplebaseclassesintheparenthesesafteritsnameinthedeclaration.

Don’tconfuseclassinstancesandderivedclasses–aninstanceisacopyofaclass,whereasaderivedclassisanewclassthatinheritspropertiesofthebaseclassfromwhichitisderived.

OverridingmethodsAmethodcanbedeclaredinaderivedclasstooverrideamatchingmethodinthebaseclass–ifbothmethoddeclarationshavethesamenameandthesamenumberoflistedparameters.Thiseffectivelyhidesthebaseclassmethodasitbecomesinaccessibleunlessitiscalledexplicitly,usingthebaseclassnameforidentification.

Whereamethodinabaseclasssuppliesadefaultparametervaluethiscanbeusedinanexplicitcalltothebasemethodoralternativevaluescanbesuppliedbyoverridingmethods.

Person.py

CreateanewclassfilethatdeclaresabaseclasswithainitializermethodtosetaninstancevariableandasecondmethodtodisplaythatvariablevalueclassPerson:

‘‘’AbaseclasstodefinePersonproperties.’’’def__init__(self,name):

self.name=namedefspeak(self,msg=‘(CallingTheBaseClass)’):

print(self.name,msg)

Man.py

Next,createaclassfilethatdeclaresaderivedclasswithamethodthatoverridesthesecondbaseclassmethodfromPersonimport*

‘‘’AderivedclasstodefineManproperties.’’’

classMan(Person):defspeak(self,msg):

print(self.name,‘:\n\tHello!’,msg)

Hombre.py

Now,createanotherclassfilethatalsodeclaresaderivedclasswithamethodthatonceagainoverridesthesamemethodinthebaseclassfromPersonimport*

‘‘’AderivedclasstodefineHombreproperties.’’’classHombre(Person):

defspeak(self,msg):print(self.name,‘:\n\tHola!’,msg)

override.py

SavethethreeclassfilesthenstartanewprogrambymakingfeaturesofbothderivedclassesavailablefromManimport*

fromHombreimport*

Next,createaninstanceofeachderivedclass,initializingthe“name”instancevariableattributeguy_1=Man(‘Richard’)guy_2=Hombre(‘Ricardo’)

Now,calltheoverridingmethodsofeachderivedclass,assigningdifferentvaluestothe“msg”parameterguy_1.speak(‘It\’sabeautifulevening.\n’)

guy_2.speak(‘Esunatardehermosa.\n’)

Finally,explicitlycallthebaseclassmethod,passingareferencetoeachderivedclass–butnoneforthe“msg”variablesoitsdefaultvaluewillbeusedPerson.speak(guy_1)Person.speak(guy_2)

Savethenruntheprogram–toseeoutputfromoverridingandbaseclassmethods

Man–Richard

Hombre–Ricardo

Themethoddeclarationinthederivedclassmustexactlymatchthatinthebaseclasstooverrideit.

ApplyingsenseThethreecornerstonesofObjectOrientedProgramming(OOP)areencapsulation,inheritance,andpolymorphism.ExamplesearlierinthischapterhavedemonstratedhowdatacanbeencapsulatedwithinaPythonclass,andhowderivedclassesinheritthepropertiesoftheirbaseclass.Thisexampleintroducesthefinalcornerstoneprincipleofpolymorphism.

Theterm“polymorphism”(fromGreek,meaning“manyforms”)describestheabilitytoassignadifferentmeaning,orpurpose,toanentityaccordingtoitscontext.

InPython,the+characterentitycanbedescribedaspolymorphicbecauseitrepresentseitherthearithmeticaladditionoperator,inthecontextofnumericaloperands,orthestringconcatenationoperatorinthecontextofcharacteroperands.

Perhapsmoreimportantly,PythonclassmethodscanalsobepolymorphicbecausethePythonlanguageuses“ducktyping”–meaning…ifitwalkslikeaduck,swimslikeaduck,andquackslikeaduck,thenthatbirdisreckonedtobeaduck.

Inaduck-typedlanguageyoucancreateafunctiontotakeanobjectofanytypeandcallthatobject’smethods.Iftheobjectdoesindeedhavethecalledmethods(isreckonedtobeaduck)theyareexecuted,otherwisethefunctionsignalsarun-timeerror.

Like-namedmethodsofmultipleclassescanbecreatedandinstancesofthoseclasseswillexecutetheassociatedversion.

Duck.py

CreateanewclassfilethatdeclaresaclasswithmethodstodisplaystringsuniquetotheclassclassDuck:

deftalk(self):print(‘\nDuckSays:Quack!’)

defcoat(self):print(‘DuckWears:Feathers’)

Mouse.py

Next,createaclassfilethatdeclaresaclasswithlike-namedmethodsbuttodisplaystringsuniquetothisclassclassMouse:

deftalk(self):print(‘\nMouseSays:Squeak!’)

defcoat(self):print(‘MouseWears:Fur’)

polymorph.py

SavethetwoclassfilesthenstartanewprogrambymakingfeaturesofbothclassesavailablefromDuckimport*

fromMouseimport*

Next,defineafunctionthatacceptsanysingleobjectasitsparameterandattemptstocallmethodsofthatobjectdefdescribe(object):

object.talk()

object.coat()

Now,createaninstanceobjectofeachclass

donald=Duck()mickey=Mouse()

Finally,addstatementstocallthefunctionandpasseachinstanceobjecttoitasanargumentdescribe(donald)

describe(mickey)

Savethenruntheprogram–toseethemethodsoftheassociatedversionsgetcalled

Duck–donald

Mouse–mickey

Aclasscanhaveonlyonemethodwithagivenname–methodoverloadingisnotsupportedinPython.

ObjectOrientedProgrammingwithPythonallowsdataencapsulation,inheritance,andpolymorphism.Baseclassmethodscanbeoverriddenbylike-namedmethodsinderivedclasses.Pythondoesnot,however,supportthetechniqueof“overloading”foundinotherlanguages–inwhichmethodsofthesamenamecanbecreatedwithdifferentparameterlistsinasingleclass.

Summary• Programmingobjectsarelikevirtualrepresentationsofreal-worldobjectsdescribing

attributesandbehaviorsina“class”

• Aclassisadatastructureprototypedescribingobjectpropertieswithitsmethodsandattributemembers

• Eachclassdeclarationbeginswiththeclasskeywordandisfollowedbyanindentedcodeblockthatmaycontainaclassdocumentstring,classvariables,andclassmethods

• Classvariableshaveglobalscopebutinstancevariables(declaredwithinmethoddefinitions)haveonlylocalscope

• Instancevariablesencapsulatedatasecurelyinaclassstructureandareinitializedwhenaclassinstanceiscreated

• Propertiesofaclassarereferencedbydotnotationandareaddressedinternallyusingtheselfprefix

• Aclassinstanceisacopyoftheprototypethatautomaticallycallsits__init__()methodwhentheinstanceisfirstcreated

• Anattributeofaclasscanbeadded,modified,orremovedusingdotnotationormanipulatedusingthebuilt-infunctionsgetattr(),hasattr(),setattr(),anddelattr()

• ThenameofattributesautomaticallysuppliedbyPythonbeginwithanunderscorecharactertonotionallyindicateprivacy

• Aderivedclassinheritsthemethodandattributemembersoftheparentbaseclassfromwhichitisderived

• Thedeclarationofaderivedclassmuststatethenameofitsparentbaseclassinparenthesesafteritsownclassname

• Amethodofaderivedclasscanoverrideamatchingmethodofthesamenameinitsparentbaseclass

• Amethodofabaseclasscanbecalledexplicitlyusingthebaseclassnameforidentification

• Pythonisaduck-typedlanguagethatsupportspolymorphismforlike-namedmethodsofmultipleclasses

• ThethreecornerstonesofObjectOrientedProgramming(OOP)areEncapsulation,Inheritance,andPolymorphism

11Buildinginterfaces

Thischapterdemonstrateshowtocodegraphicalwindowedprograms.

LaunchinginterfacesRespondingbuttonsDisplayingmessagesGatheringentriesListingoptionsPollingradiosCheckingboxesAddingimagesSummary

LaunchinginterfacesThestandardPythonmodulethatyoucanusetocreategraphicalapplicationsiscalled“tkinter”–atoolkittointerfacewiththesystemGUI(GraphicalUserInterface).

Thetkintermodulecanbeimportedintoaprogramlikeanyothermoduletoprovideattributesandmethodsforwindowedapps.EverytkinterprogrammustbeginbycallingtheTk()constructortocreateawindowobject.Thewindow’ssizecanoptionallybespecifiedasa‘widthxheight’stringargumenttothewindowobject’sgeometry()method.Similarly,thewindow’stitlecanbespecifiedasa‘title’stringargumenttothewindowobject’stitle()method.Ifnotspecifieddefaultsizeandtitlevalueswillbeused.

TherecanbeonlyonecalltotheTk()constructoranditmustbeatthestartoftheprogramcode.

Everytkinterprogrammustalsocallthewindowobject’smainloop()methodtocaptureevents,suchaswhentheuserclosesthewindowtoquittheprogram.Thisloopshouldappearattheendoftheprogramasitalsohandleswindowupdatesthatmaybeimplementedduringexecution.

Withtkinter,allthegraphicalcontrolsthatcanbeincludedintheapplicationwindow,suchasbuttonsorcheckboxes,arereferredtoas“widgets”.Perhapsthesimplestwidgetisanon-interactivelabelobjectthatmerelydisplaystextoranimageintheappinterface.Alabelobjectcanbecreatedbyspecifyingthewindowobject’snameandtext=’string’asargumentstoaLabel()constructor.

Oncecreated,eachwidget,suchasalabel,mustthenbeaddedtothewindowusingoneofthese“geometrymanager”methods:

• pack()–placesthewidgetagainstaspecifiedsideofthewindowusingTOP,BOTTOM,LEFT,orRIGHTconstantvaluesspecifiedtoitsside=argument

• place()–placesthewidgetatXYcoordinatesinthewindowusingnumericalvaluesspecifiedtoitsx=andy=arguments

• grid()–placesthewidgetinacellwithinthewindowusingnumericalvaluesspecifiedtorow=andcolumn=arguments

Thegrid()geometrymanagermethodisdemonstratedintheexamplehere.

Optionally,thepack()methodmayincludeafillargumenttoexpandthewidgetinavailable

space.Forexample,withfill=‘x’.Alternatively,thepack()methodmayincludepadxandpadyargumentstoexpandthewidgetalonganaxisaspecifiedamount.

window.py

Startanewprogramwithastatementtomakethe“tkinter”moduleGUImethodsandattributesavailablefromtkinterimport*

Next,addastatementtocalluponaconstructortocreateawindowobject

window=Tk()

Now,addastatementtospecifyatitleforthiswindow

window.title(‘LabelExample’)

Then,addastatementtocalluponaconstructortocreatealabelobject

label=Label(window,text=‘HelloWorld!’)

Usethepackertoaddthelabeltothewindowwithbothhorizontalandverticalpaddingforpositioninglabel.pack(padx=200,pady=50)

Finally,addthemandatorystatementtomaintainthewindowbycapturingevents

window.mainloop()

Savethenruntheprogram–toseeawindowappearcontainingalabelwidget

Widgetswillnotappearinthewindowwhenrunningtheprogramunlesstheyhavebeenaddedwithageometrymanager.

RespondingbuttonsAButtonwidgetprovidesagraphicalbuttoninanapplicationwindowthatmaycontaineithertextoranimagetoconveythebutton’spurpose.AbuttonobjectiscreatedbyspecifyingthewindownameandoptionsasargumentstoaButton()constructor.Eachoptionisspecifiedasanoption=valuepair.Thecommandoptionmustalwaysspecifythenameofafunctionormethodtocallwhentheuserclicksthatbutton.Themostpopularoptionsarelistedbelowtogetherwithabriefdescription:

Option: Description:

activebackground Backgroundcolorwhenthecursorisover

activeforeground Foregroundcolorwhenthecursorisover

bd Borderwidthinpixels(defaultis2)

bg Backgroundcolor

command Functiontocallwhenclicked

fg Foregroundcolor

font Fontforbuttonlabel

height Buttonheightintextlines,orpixelsforimages

highlightcolor Bordercolorwheninfocus

image Imagetobedisplayedinsteadoftext

justify MultipletextlinesasLEFT,CENTER,orRIGHT

padx Horizontalpadding

pady Verticalpadding

relief BorderstyleofSUNKEN,RIDGE,RAISEDorGROOVE

state EnabledstatusofNORMALorDISABLED

underline Indexnumberintextofcharactertounderline

width Buttonwidthinletters,orpixelsforimages

wraplength Lengthatwhichtowraptext

Youcanalsocallabutton’sinvoke()methodto,inturn,callthefunctionnominatedtoitscommandoption.Thevaluesassignedtootheroptionsdeterminethewidget’sappearance.Thesecanbealteredbyspecifyinganewoption=valuepairasanargumenttothewidget’sconfigure()method.Additionally,acurrentoptionvaluecanberetrievedbyspecifyingitsnameasastringargumenttothewidget’scget()method.

button.py

StartanewprogrambymakingGUIfeaturesavailablethencreateawindowandspecifyatitlefromtkinterimport*

window=Tk()window.title(‘ButtonExample’)

Next,createabuttontoexittheprogramwhenclicked

btn_end=Button(window,text=‘Close’,command=exit)

Now,addafunctiontotogglethewindow’sbackgroundcolorwhenanotherbuttongetsclickeddeftog():

ifwindow.cget(‘bg’)==‘yellow’:window.configure(bg=‘gray’)

else:window.configure(bg=‘yellow’)

Then,createabuttontocallthefunctionwhenclicked

btn_tog=Button(window,text=‘Switch’,command=tog)

Addthebuttonstothewindowwithpositionalpadding

btn_end.pack(padx=150,pady=20)

btn_tog.pack(padx=150,pady=20)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethenruntheprogramandclickthebutton–toseethewindow’sbackgroundcolorchange

Onlythefunctionnameisspecifiedtothecommandoption.Donotaddtrailingparenthesesintheassignment.

The‘gray’coloristheoriginaldefaultcolorofthewindow.

DisplayingmessagesAprogramcandisplaymessagestotheuserbycallingmethodsprovidedinthe“tkinter.messagebox”module.Thismustbeimportedseparatelyanditslengthynamecanusefullybeassignedashortaliasbyanimportasstatement.

Amessageboxiscreatedbysupplyingaboxtitleandthemessagetobedisplayedasthetwoargumentstooneofthesemethods:

Method: Icon: Buttons:

showinfo() OK

showwarning() OK

showerror() OK

askquestion() Yes(returnsthestring‘yes’)andNo(returnsthestring‘no’)

askokcancel() OK(returns1)andCancel

askyesno() Yes(returns1)andNo

askretrycancel() Retry(returns1)andCancel

Onlytheaskquestion()methodreturnstwovalues–theaskyesno()NobuttonandbothCancelbuttonsreturnnothing.

ThosemethodsthatproduceamessageboxcontainingasingleOKbuttonreturnnovaluewhenthebuttongetsclickedbytheuser.Thosethatdoreturnavaluecanbeusedtoperformconditionalbranchingbytestingthatvalue.

message.py

StartanewprogrambymakingGUIfeaturesavailableandmessageboxfeaturesavailableasashortaliasfromtkinterimport*

importtkinter.messageboxasbox

Next,createawindowobjectandspecifyatitle

window=Tk()window.title(‘MessageBoxExample’)

Addafunctiontodisplayvariousmessageboxes

defdialog():

var=box.askyesno(‘MessageBox’,‘Proceed?’)ifvar==1:

box.showinfo(‘YesBox’,‘Proceeding…’)else:

box.showwarning(‘NoBox’,‘Canceling…’)

Then,createabuttontocallthefunctionwhenclicked

btn=Button(window,text=‘Click’,command=dialog)

Addthebuttontothewindowwithpositionalpadding

btn.pack(padx=150,pady=50)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethefilethenruntheprogramandclickthebutton–toseethemessageboxesappear

Optionscanbeaddedasathirdargumenttothesemethodcalls.Forexample,addtype=’abortretryignore’togetthreebuttons.

GatheringentriesAnEntrywidgetprovidesasingle-lineinputfieldinanapplicationwheretheprogramcangatherentriesfromtheuser.Anentryobjectiscreatedbyspecifyingthenameofitsparentcontainer,suchasawindoworframename,andoptionsasargumentstoanEntry()constructor.Eachoptionisspecifiedasanoption=valuepair.Popularoptionsarelistedbelowtogetherwithabriefdescription:

Option: Description:

bd Borderwidthinpixels(defaultis2)

bg Backgroundcolor

fg Foregroundcolorusedtorenderthetext

font Fontforthetext

highlightcolor Bordercolorwheninfocus

selectbackground Backgroundcolorofselectedtext

selectforeground Foregroundcolorofselectedtext

show Hidepasswordcharacterswithshow=’*’

state EnabledstatusofNORMALorDISABLED

width Entrywidthinletters

Multiplewidgetscanbegroupedinframesforbetterpositioning.AframeobjectiscreatedbyspecifyingthenameofthewindowtoaFrame()constructor.Theframe’snamecanthenbespecifiedasthefirstargumenttothewidgetconstructorstoidentifyitasthatwidget’scontainer.

Whenactuallyaddingwidgetstotheframe,youcanspecifywhichsidetopackthemtointheframewithTOP,BOTTOM,LEFT,orRIGHTconstants.Forexample,entry.pack(side=LEFT).

UsetheTextwidgetinsteadofanEntrywidgetifyouwanttoallowtheuserto

entermultiplelinesoftext.

Typically,anentrywidgetwillappearalongsidealabeldescribingthetypeofinputexpectedtherefromtheuser,oralongsideabuttonwidgetthattheusercanclicktoperformsomeactiononthedatatheyhaveentered,sopositioninginaframeisideal.

Datacurrentlyenteredintoanentrywidgetcanberetrievedbytheprogramusingthatwidget’sget()method.

entry.py

StartanewprogrambymakingGUIfeaturesavailableandmessageboxfeaturesavailableasashortaliasfromtkinterimport*

importtkinter.messageboxasbox

Next,createawindowobjectandspecifyatitle

window=Tk()window.title(‘EntryExample’)

Now,createaframetocontaininganentryfieldforinput

frame=Frame(window)

entry=Entry(frame)

Then,addafunctiontodisplaydatacurrentlyentered

defdialog():box.showinfo(‘Greetings’,‘Welcome’+entry.get())

Now,createabuttontocallthefunctionwhenclicked

btn=Button(frame,text=‘EnterName’,command=dialog)

Addthebuttonandentrytotheframeatsetsides

btn.pack(side=RIGHT,padx=5)

entry.pack(side=LEFT)frame.pack(padx=20,pady=20)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethefileandruntheprogram,thenenteryournameandclickthebutton–toseeagreetingmessageappear

UseaLabelwidgetinsteadofanEntrywidgetifyouwanttodisplaytextthattheusercannotedit.

ListingoptionsAListboxwidgetprovidesalistofitemsinanapplicationfromwhichtheusercanmakeaselection.Alistboxobjectiscreatedbyspecifyingthenameofitsparentcontainer,suchasawindoworframename,andoptionsasargumentstoaListbox()constructor.Popularoptionsarelistedbelowtogetherwithabriefdescription:

Option: Description:

bd Borderwidthinpixels(defaultis2)

bg Backgroundcolor

fg Foregroundcolorusedtorenderthetext

font Fontforthetext

height Numberoflinesinlist(defaultis10)

selectbackground Backgroundcolorofselectedtext

selectmode SINGLE(thedefault)orMULTIPLEselections

width Listboxwidthinletters(defaultis20)

yscrollcommand Attachtoaverticalscrollbar

Withtkinter,ascrollbarisaseparatewidgetthatcanbeattachedtoListbox,Text,CanvasandEntrywidgets.

Itemsareaddedtothelistboxbyspecifyingalistindexnumberandtheitemstringasargumentstoitsinsert()method.

Youcanretrieveanyitemfromalistboxbyspecifyingitsindexnumberwithintheparenthesesofitsget()method.Usefully,alistboxalsohasacurselection()methodthatreturnstheindexnumberofthecurrentlyselecteditem,sothiscanbesuppliedastheargumenttoitsget()methodtoretrievethecurrentselection.

listbox.py

StartanewprogrambymakingGUIfeaturesavailableandmessageboxfeaturesavailableasashortaliasfromtkinterimport*

importtkinter.messageboxasbox

Next,createawindowobjectandspecifyatitle

window=Tk()window.title(‘ListboxExample’)

Now,createaframetocontainwidgets

frame=Frame(window)

Createalistboxwidgetofferingthreelistitems

listbox=Listbox(frame)

listbox.insert(1,‘HTML5ineasysteps’)listbox.insert(2,‘CSS3ineasysteps’)

listbox.insert(3,‘JavaScriptineasysteps’)

Next,addafunctiontodisplayalistboxselection

defdialog():box.showinfo(‘Selection’,‘YourChoice:’+\

listbox.get(listbox.curselection()))

Now,createabuttontocallthefunctionwhenclicked

btn=Button(frame,text=‘Choose’,command=dialog)

Then,addthebuttonandlistboxtotheframeatsetsides

btn.pack(side=RIGHT,padx=5)listbox.pack(side=LEFT)

frame.pack(padx=30,pady=30)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethefileandruntheprogram,thenselectanoptionandclickthebutton–toseeyourselectionconfirmed

IftheselectmodeissettoMULTIPLE,thecurselection()methodreturnsatupleoftheselectedindexnumbers.

PollingradiosARadiobuttonwidgetprovidesasingleiteminanapplicationthattheusermayselect.Whereanumberofradiobuttonsaregroupedtogethertheusermayonlyselectanyoneiteminthegroup.Withtkinter,radiobuttonobjectsaregroupedtogetherwhentheynominatethesamecontrolvariableobjecttoassignavaluetouponselection.AnemptystringvariableobjectcanbecreatedforthispurposeusingtheStringVar()constructororanemptyintegervariableobjectusingtheIntVar()constructor.

Youcannotusearegularvariabletostorevaluesassignedfromaradiobuttonselection–itmustbeanobject.

AradiobuttonobjectiscreatedbyspecifyingfourargumentstoaRadiobutton()constructor:

• Nameoftheparentcontainer,suchastheframename

• Textforadisplaylabel,specifiedasatext=textpair

• Controlvariableobject,specifiedasavariable=variablepair

• Valuetobeassigned,specifiedasavalue=valuepair

Eachradiobuttonobjecthasaselect()methodthatcanbeusedtospecifyadefaultselectioninagroupofradiobuttonswhentheprogramstarts.Astringvalueassignedbyselectingaradiobuttoncanberetrievedfromastringvariableobjectbyitsget()method.

radio.py

StartanewprogrambymakingGUIfeaturesavailableandmessageboxfeaturesavailableasashortaliasfromtkinterimport*

importtkinter.messageboxasbox

Next,createawindowobjectandspecifyatitle

window=Tk()window.title(‘RadioButtonExample’)

Now,createaframetocontainwidgets

frame=Frame(window)

Then,constructastringvariableobjecttostoreaselection

book=StringVar()

Next,createthreeradiobuttonwidgetswhosevaluewillbeassignedtothestringvariableuponselectionradio_1=Radiobutton(frame,text=‘HTML5’,\

variable=book,value=‘HTML5ineasysteps’)radio_2=Radiobutton(frame,text=‘CSS3’,\

variable=book,value=‘CSS3ineasysteps’)radio_3=Radiobutton(frame,text=‘JS’,\

variable=book,value=‘JavaScriptineasysteps’)

Now,addastatementtospecifywhichradiobuttonwillbeselectedbydefaultwhentheprogramstartsradio_1.select()

Then,addafunctiontodisplayaradiobuttonselectionandabuttontocallthisfunctiondefdialog():

box.showinfo(‘Selection’,\

‘YourChoice:\n’+book.get())btn=Button(frame,text=‘Choose’,command=dialog)

Addthepushbuttonandradiobuttonstotheframe

btn.pack(side=RIGHT,padx=5)

radio_1.pack(side=LEFT)radio_2.pack(side=LEFT)

radio_3.pack(side=LEFT)frame.pack(padx=30,pady=30)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethefileandruntheprogram,thenchooseanoptionandclickthebutton–toseeyourchoiceconfirmed

ARadiobuttonobjecthasadeselect()methodthatcanbeusedtocancelaselectionprogramatically.

CheckingboxesACheckbuttonwidgetprovidesasingleiteminanapplicationthattheusermayselect.Whereanumberofcheckbuttonsappeartogethertheusermayselectoneormoreitems.Checkbuttonobjectsnominateanindividualcontrolvariableobjecttoassignavaluetowhethercheckedorunchecked.AnemptystringvariableobjectcanbecreatedforthisusingtheStringVar()constructororanemptyintegervariableobjectusingtheIntVar()constructor.

AcheckbuttonobjectiscreatedbyspecifyingfiveargumentstoaCheckbutton()constructor:

• Nameoftheparentcontainer,suchastheframename

• Textforadisplaylabel,asatext=textpair

• Controlvariableobject,asavariable=variablepair

• Valuetoassignifchecked,asanonvalue=valuepair

• Valuetoassignifunchecked,asanoffvalue=valuepair

Anintegervalueassignedbyacheckbuttoncanberetrievedfromaintegervariableobjectbyitsget()method.

check.py

StartanewprogrambymakingGUIfeaturesavailableandmessageboxfeaturesavailableasashortaliasfromtkinterimport*

importtkinter.messageboxasbox

Next,createawindowobjectandspecifyatitle

window=Tk()window.title(‘CheckButtonExample’)

Now,createaframetocontainwidgets

frame=Frame(window)

Then,constructthreeintegervariableobjectstostorevalues

var_1=IntVar()

var_2=IntVar()var_3=IntVar()

Createthreecheckbuttonwidgetswhosevalueswillbeassignedtotheinteger

variablewhethercheckedornotbook_1=Checkbutton(frame,text=‘HTML5’,\

variable=var_1,onvalue=1,offvalue=0)book_2=Checkbutton(frame,text=‘CSS3’,\

variable=var_2,onvalue=1,offvalue=0)book_3=Checkbutton(frame,text=‘JS’,\

variable=var_3,onvalue=1,offvalue=0)

Next,addafunctiontodisplayacheckbuttonselection

defdialog():s=‘YourChoice:’

ifvar_1.get()==1:s+=‘\nHTML5ineasysteps’ifvar_2.get()==1:s+=‘\nCSS3ineasysteps’

ifvar_3.get()==1:s+=‘\nJavaScriptineasysteps’box.showinfo(‘Selection’,s)

Now,createabuttontocallthefunctionwhenclicked

btn=Button(frame,text=‘Choose’,command=dialog)

Then,addthepushbuttonandcheckbuttonstotheframe

btn.pack(side=RIGHT,padx=5)

book_1.pack(side=LEFT)book_2.pack(side=LEFT)

book_3.pack(side=LEFT)frame.pack(padx=30,pady=30)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethefileandruntheprogram,thencheckboxesandclickthebutton–toseeyourselectionconfirmed

ACheckbuttonobjecthasselect()anddeselect()methodsthatcanbeusedtoturnthestateonoroff.Forexample,check_1.select().

ThestateofanyCheckbuttonobjectcanbereversedbycallingitstoggle()method.

AddingimagesWiththetkintermodule,imagesinGIForPGM/PPMfileformatscanbedisplayedonLabel,Button,TextandCanvaswidgetsusingthePhotoImage()constructortocreateimageobjects.Thissimplyrequiresasinglefile=argumenttospecifytheimagefile.Interestingly,italsohasasubsample()methodthatcanscaledownaspecifiedimagebystatingasamplevaluetox=andy=arguments.Forexample,valuesofx=2,y=2sampleseverysecondpixel–sotheimageobjectishalf-sizeoftheoriginal.

OnceanimageobjecthasbeencreateditcanbeaddedtoaLabelorButtonconstructorstatementbyanimage=option.

ThePhotoImageclassalsohasazoom()methodthatwilldoubletheimagesizewiththesamex=2,y=2values.Textobjectshaveanimage_create()methodwithwhichtoembedanimageintothetextfield.Thisrequirestwoargumentstospecifylocationandimage=.Forexample,‘1.0’specifiesthefirstlineandfirstcharacter.

Canvasobjectshaveacreate_image()methodthatrequirestwoargumentstospecifylocationandimage=.Herethelocationsetsthex,ycoordinatesonthecanvasatwhichtopainttheimage.

image.py

StartanewprogrambymakingGUImethodsandattributesavailablethencreateawindowobjectandspecifyatitlefromtkinterimport*

window=Tk()window.title(‘ImageExample’)

Now,createanimageobjectfromalocalimagefile

img=PhotoImage(file=‘python.gif’)

python.gif(200x200)

Then,createalabelobjecttodisplaytheimageaboveacoloredbackground

label=Label(window,image=img,bg=‘yellow’)

Createahalf-sizeimageobjectfromthefirstimageobject

small_img=PhotoImage.subsample(img,x=2,y=2)

Now,createabuttontodisplaythesmallimage

btn=Button(window,image=small_img)

Createatextfieldandembedthesmallimagetheninsertsometextafterit

txt=Text(window,width=25,height=7)

txt.image_create(‘1.0’,image=small_img)txt.insert(‘1.1’,‘PythonFun!’)

Createacanvasandpaintthesmallimageaboveacoloredbackgroundthenpaintadiagonallineoverthetopofitcan=\

Canvas(window,width=100,height=100,bg=‘cyan’)can.create_image((50,50),image=small_img)

can.create_line(0,0,100,100,width=25,fill=‘yellow’)

Then,addthewidgetstothewindow

label.pack(side=TOP)btn.pack(side=LEFT,padx=10)

txt.pack(side=LEFT)can.pack(side=LEFT,padx=10)

Finally,addthelooptocapturethiswindow’sevents

window.mainloop()

Savethefilethenruntheprogram–toseetheimageontheLabel,Button,TextandCanvaswidgets

NoticethattheTextmethodisimage_create()buttheCanvasmethodiscreate_image()–similaryetdifferent.

TextandCanvaswidgetsarebothpowerfulandflexible–discovermoreonlineatdocs.python.org/3.3/library/tkinter.html

Summary• ThetkintermodulecanbeimportedintoaPythonprogramtoprovideattributesand

methodsforwindowedapplications

• EverytkinterprogrammustbeginbycallingTk()tocreateawindowandcallitsmainloop()methodtocaptureevents

• Thewindowobject’stitleisspecifiedbyitstitle()method

• AlabelwidgetiscreatedbyspecifyingthenameofitsparentcontaineranditstextasargumentstotheLabel()constructor

• Widgetscanbeaddedtoanapplicationusingthepack(),grid()orplace()geometrymanagers

• Abuttonwidgetiscreatedbyspecifyingthenameofitsparentcontainer,itstext,andthenameofafunctiontocallwhentheuserpushesit,asargumentstotheButton()constructor

• Thetkinter.messageboxmodulecanbeimportedintoaPythonprogramtoprovideattributesandmethodsformessageboxes

• Messageboxesthatasktheusertomakeachoicereturnavaluetotheprogramforconditionalbranching

• TheFrame()constructorcreatesacontainerinwhichmultiplewidgetscanbegroupedforbetterpositioning

• TheEntry()constructorcreatesasinglelinetextfieldwhosecurrentcontentscanberetrievedbyitsget()method

• ItemsareaddedtoaListboxobjectbyitsinsert()methodandretrievedbyspecifyingtheirindexnumbertoitsget()method

• RadiobuttonandCheckbuttonobjectsstorevaluesintheStringVarorIntVarobjectnominatedbytheirvariableattribute

• ThePhotoImage()constructorcreatesanimageobjectthathasasubsample()methodwhichcanscaledowntheimage

• ImagescanbeaddedtoButtonandLabelobjects,embeddedinTextobjects,andpaintedonCanvasobjects

12Developingapps

Thischapterbringstogetherelementsfrompreviouschapterstodevelopawindowedapplication.

GeneratingrandomsPlanningneedsDesigninglayoutAssigningstaticsLoadingdynamicsAddingfunctionalityTestingprogramsDeployingapplicationsSummary

GeneratingrandomsThegraphicalapplicationdevelopedonsubsequentpagesofthisbookwillgeneratesixrandomnumberswithinaspecificrange.Initially,itsfunctionalitycanbedevelopedasaconsoleapplicationthentransferredlatertoillustratehowitcanbeappliedtographicalwidgetcomponents.

ThestandardPythonlibraryhasarandommodulethatprovidesmethodstogeneratepseudo-randomnumbers.Thecurrentsystemtimeisusedbydefaultto“seed”therandomgeneratorwheneveritgetsinitialized–soitdoesnotrepeatitsselections.

Apseudo-randomfloating-pointnumberfrom0.0to1.0canbegeneratedbycallingtherandom()methodfromtherandommodule.Therangeofgeneratednumberscanbemodifiedusingthe*multiplicationoperatortospecifyamaximumvalueandcanberoundeddowntointegervaluesusingthebuilt-inint()function.Forexample,togenerateanintegerwithintherangeofzerotonine:int(random.random()*10)

Ortogenerateawholenumberwithintherangeof1to10:int(random.random()*10)+1

Floating-pointnumberscastfromthefloatdatatypetotheintdatatypebythebuilt-inint()functiongettruncatedatthedecimalpoint.Thisstatementcouldbeusedinalooptogeneratemultiplerandomintegerswithinagivenrangebutanynumbermayberepeatedinthatoutput–thereisnoguaranteeduniqueness.Instead,multipleuniquerandomintegerswithinagivenrangecanbegeneratedbythesample()methodfromtherandommodule.Thisrequirestwoargumentstospecifytherangeandthenumberofuniqueintegerstobereturned.Itisconvenienttousethebuilt-inrange()functiontospecifyamaximumvalue.Forexample,togeneratesixuniquenumberswithintherangeof1to9:random.sample(range(10),6)

Ortogeneratesixuniquenumberswithintherangeof1to10:random.sample(range(1,11),6)

Thistechniquecouldrepresentarandomlotteryentrybychoosing,say,sixuniquenumbersbetween1and49.

Therange()functioncanspecifystartandendvalues.Ifnostartingvalueissupplied,zeroisassumedbydefault.

sample.py

Startanewprogrambyimportingtwofunctionsfromthe“random”module

fromrandomimportrandom,sample

Next,assignarandomfloating-pointnumbertoavariablethendisplayitsvalue

num=random()

print(‘RandomFloat0.0-1.0:‘,num)

Now,multiplythefloating-pointnumberandcastittobecomeanintegerthendisplayitsvaluenum=int(num*10)print(‘RandomInteger0-9:‘,num)

Addalooptoassignmultiplerandomintegerstoalist,thendisplaythelistitems

nums=[];i=0

whilei<6:nums.append(int(random()*10)+1)

i+=1print(‘RandomMultipleIntegers1-10:’,nums)

Finally,assignmultipleuniquerandomintegerstothelistthendisplaythelistitemsnums=sample(range(1,49),6)

print(‘RandomIntegerSample1-49:‘,nums)

Savethefilethenruntheprogramseveraltimes–toseethegeneratedrandomnumbers

Therandom.sample()functionreturnsalistbutdoesnotactuallyreplaceanyelementsinthespecifiedrange.

PlanningneedsWhencreatinganewgraphicalapplicationitisusefultofirstspendsometimeplanningitsdesign.Clearlydefinetheprogram’sprecisepurpose,decidewhatapplicationfunctionalitywillberequired,thendecidewhatinterfacewidgetswillbeneeded.

Aplanforasimpleapplicationtopicknumbersforalotteryentrymightlooklikethis:

Programpurpose

• Theprogramwillgenerateaseriesofsixuniquerandomnumbersintherange1-49andhavetheabilitytobereset

Functionalityrequired

• Afunctiontogenerateanddisplaysixuniquerandomnumbers

• Afunctiontoclearthelastsixrandomnumbersfromdisplay

Interfacewidgetsneeded

• Onenon-resizablewindowtocontainallotherwidgetsandtodisplaytheapplicationtitle.

• OneLabelwidgettodisplayastaticapplicationlogoimage–justtoenhancetheappearanceoftheinterface.

• SixLabelwidgetstodynamicallydisplaythegeneratedseriesofuniquerandomnumbers–onenumberperLabel.

• OneButtonwidgettogenerateanddisplaythenumbersintheLabelwidgetswhenthisButtongetsclicked.ThisButtonwillnotbeenabledwhenthenumbersareondisplay.

• OneButtonwidgettoclearthenumbersondisplayintheLabelwidgetswhenthisButtongetsclicked.ThisButtonwillnotbeenabledwhenthenumbersarenotondisplay.

Havingestablishedaprogramplanmeansyoucannowproducetheapplicationbasicsbycreatingallthenecessarywidgets.

TogglethevalueofaButtonwidget’sstatepropertyfromNORMALtoDISABLEDtosteertheuser–inthiscasetheapplicationmustberesetbeforea

furtherseriesofuniquerandomnumberscanbegenerated.

lotto(widgets).py

Startanewprogrambyimportingallfeaturesfromthe“tkinter”module

#Widgets:

fromtkinterimport*

Next,addstatementstocreateawindowobjectandanimageobject

window=Tk()img=PhotoImage(file=‘lotto.gif’)

lotto.gif

Now,addstatementstocreateallthenecessarywidgets

imgLbl=Label(window,image=img)

label1=Label(window,relief=‘groove’,width=2)label2=Label(window,relief=‘groove’,width=2)

label3=Label(window,relief=‘groove’,width=2)label4=Label(window,relief=‘groove’,width=2)

label5=Label(window,relief=‘groove’,width=2)label6=Label(window,relief=‘groove’,width=2)

getBtn=Button(window)resBtn=Button(window)

Then,addthewidgetstothewindowusingthegridlayoutmanager–readytoreceiveargumentstospecifyhowthewidgetsshouldbepositionedatthedesignstagenext#Geometry:

imgLbl.grid()label1.grid()

label2.grid()label3.grid()

label4.grid()label5.grid()

label6.grid()

getBtn.grid()

resBtn.grid()

Finally,addaloopstatementtosustainthewindow

#Sustainwindow:window.mainloop()

Savethefilethenruntheprogram–toseethewindowappearcontainingallthenecessarywidgets

Thereliefpropertyspecifiesaborderstyleandthewidthpropertyspecifiesthelabelwidthincharacternumbers.

DesigninglayoutHavingcreatedallthenecessarywidgets,onthepreviouspage,youcannowdesigntheinterfacelayoutbyaddingargumentstospecifyhowthewidgetsshouldbepositioned.AhorizontaldesignwillpositionthelogoLabelontheleft,andonitsrightallsixotherLabelsinarowwithbothButtonsbelowthis.Thegridlayoutmanager,whichpositionswidgetsinrowsandcolumns,caneasilyproducethisdesignbyallowingthelogoLabeltospanarowcontainingallsixotherLabelsandalsoarowcontainingbothButtons.OneButtoncanspanfourcolumnsandtheotherButtoncanspantwocolumns,arrangedlikethis:

Thegridlayoutmanager’srowspanandcolumnspanpropertiesworkliketheHTMLrowspanandcolspantablecellattributes.

lotto(layout).py

Edittheprogramstartedonthepreviouspage–firstlybypositioningtheLabelcontainingthelogointhefirstcolumnofthefirstrow,andhaveitspanacrossthesecondrow#Geometry:

imgLbl.grid(row=1,column=1,rowspan=2)

Next,positionaLabelinthesecondcolumnofthefirstrowandadd10pixelsofpaddingtoitsleftandrightlabel1.grid(row=1,column=2,padx=10)

Now,positionaLabelinthethirdcolumnofthefirstrowandadd10pixelsofpaddingtoitsleftandrightlabel2.grid(row=1,column=3,padx=10)

PositionaLabelinthefourthcolumnofthefirstrowandadd10pixelsofpaddingtoitsleftandrightlabel3.grid(row=1,column=4,padx=10)

PositionaLabelinthefifthcolumnofthefirstrowandadd10pixelsofpaddingtoitsleftandrightlabel4.grid(row=1,column=5,padx=10)

PositionaLabelinthesixthcolumnofthefirstrowandadd10pixelsofpaddingtoitsleftandrightlabel5.grid(row=1,column=6,padx=10)

PositionaLabelintheseventhcolumnofthefirstrowthenadd10pixelsofpaddingtotheleftsideoftheLabeland20pixelsofpaddingtotherightsideoftheLabellabel6.grid(row=1,column=7,padx=(10,20))

Next,positionaButtoninthesecondcolumnofthesecondrowandhaveitspanacrossfourcolumnsgetBtn.grid(row=2,column=2,columnspan=4)

Now,positionaButtoninthesixthcolumnofthesecondrow,andhaveitspanacrosstwocolumnsresBtn.grid(row=2,column=6,columnspan=2)

Savethefilethenruntheprogram–toseethewindowappearcontainingallthenecessarywidgetsnowarrangedinyourgridlayoutdesign

ThewindowsizeisautomaticallyadjustedtosuitthegridcontentsandtheButtonwidgetsareautomaticallycenteredinthespannedcolumnwidth.

AdditionalpaddingtotherightoftheLabelinthefinalcolumnofthefirstrowextendsthewindowwidthtosimplycreateasmallright-handmarginarea.

TheButtonswillexpandtofitstatictextthatwillappearoneachButtonface–specifiedinthenextstage.

AssigningstaticsHavingarrangedallthenecessarywidgetsinagridlayout,onthepreviouspage,youcannowassignstaticvaluestothewidgets.Thesevalueswillnotchangeduringexecutionoftheprogram.

lotto(static).py

Modifytheprogramonthepreviouspagebyinsertinganewsectionjustbeforethefinalloopstatement,whichbeginswithastatementspecifyingawindowtitle#StaticProperties:

window.title(‘LottoNumberPicker’)

Next,addastatementtopreventtheuserresizingthewindowalongboththeXaxisandtheYaxis–thiswilldisablethewindow’s“resize”buttonwindow.resizable(0,0)

Now,addastatementtospecifytexttoappearonthefaceofthefirstButtonwidgetgetBtn.configure(text=‘GetMyLuckyNumbers’)

Then,addastatementtospecifytexttoappearonthefaceofthesecondButtonwidgetresBtn.configure(text=‘Reset’)

Savethefilethenexecutetheprogram–toseethewindownowhasatitle,itsresizebuttonisdisabled,andthebuttonshavenowbeenresizedtosuittheirtext

Thewidget’sconfigure()methodallowspropertiestobesubsequentlyaddedormodifiedaftertheyhavebeencreated.

LoadingdynamicsHavingspecifiedvaluesforstaticproperties,onthefacingpage,initialvaluescannowbespecifiedforthosepropertieswhosevalueswillchangedynamicallyduringexecutionoftheprogram.

lotto(initial).py

Modifytheprogramonthefacingpagebyinsertinganothernewsectionjustbeforethefinalloopstatement,whichspecifiesthateachsmallemptyLabelshouldinitiallydisplayanellipsis#InitialProperties:

label1.configure(text=‘…’)label2.configure(text=‘…’)

label3.configure(text=‘…’)label4.configure(text=‘…’)

label5.configure(text=‘…’)label6.configure(text=‘…’)

Next,addastatementtospecifythatthesecondButtonwidgetshouldinitiallybedisabledresBtn.configure(state=DISABLED)

Savethefilethenruntheprogram–toseeeachsmallLabelnowdisplaysanellipsisandthatthe“Reset”Buttonhasbeendisabled

ButtonstatesarerecognizedbytkinterconstantsofDISABLED(off),NORMAL(on),orACTIVE(pressed).

AddingfunctionalityHavingcreatedcodetoinitializedynamicproperties,onthepreviouspage,youcannowaddruntimefunctionalitytorespondtoclicksontheButtonwidgetsduringexecutionoftheprogram.

lotto.py

Modifytheprogramonthepreviouspagebyinsertingonemorenewsectionjustbeforethefinalloopstatement,whichbeginsbymakingthesample()functionavailablefromthe“random”module#DynamicProperties:

fromrandomimportsample

Next,defineafunctionthatgeneratesandassignssixuniquerandomnumberstothesmallLabelsandreversesthestateofbothButtonsdefpick():

nums=sample(range(1,49),6)

label1.configure(text=nums[0])label2.configure(text=nums[1])

label3.configure(text=nums[2])label4.configure(text=nums[3])

label5.configure(text=nums[4])label6.configure(text=nums[5])

getBtn.configure(state=DISABLED)resBtn.configure(state=NORMAL)

Now,defineafunctiontodisplayanellipsisoneachsmallLabelandrevertbothButtonstotheirinitialstatesdefreset():

label1.configure(text=‘…’)label2.configure(text=‘…’)

label3.configure(text=‘…’)label4.configure(text=‘…’)

label5.configure(text=‘…’)label6.configure(text=‘…’)

getBtn.configure(state=NORMAL)resBtn.configure(state=DISABLED)

Then,addstatementstonominatetherelevantfunctiontobecalledwheneach

ButtonispressedbytheusergetBtn.configure(command=pick)

resBtn.configure(command=reset)

Finally,savethefile–thecompleteprogramshouldlooklikethatshownbelow

Thesestepsprovidecomparablefunctionalitytothatoftheconsoleapplicationhere.

Itisconventiontoplaceallimportstatementsatthestartofthescriptbuttheycanappearanywhere,aslistedhere.

TestingprogramsHavingworkedthroughtheprogramplan,onthepreviouspages,thewidgetsneededandfunctionalityrequiredhavenowbeenaddedtotheapplication–soit’sreadytobetested.

Launchtheapplicationandexamineitsinitialappearance

StatictextappearsonthewindowtitlebarandontheButtonwidgets,thewindow’sresizebuttonisdisabled,thesmallLabelscontaintheirinitialellipsistextvalues,andthe“Reset”buttonisinitsinitialdisabledstate.

Next,clickthe“GetMyLuckyNumbers”Buttonwidget–toexecuteallthestatementswithinthepick()function

AseriesofnumberswithinthedesiredrangeisdisplayedandtheButtonstateshavechangedasrequired–afurtherseriesofnumberscannotbegenerateduntiltheapplicationhasbeenreset.

Makeanoteofthenumbersgeneratedinthisfirstseriesforcomparisonlater

Nonumberisrepeatedinanyseriesbecausetherandomsample()functionreturnsasetofuniquerandomintegers.

Clickthe“Reset”Buttonwidget–toexecuteallthestatementswithinthereset()functionandseetheapplicationresumeitsinitialappearanceasrequired

Clickthe“GetMyLuckyNumbers”Buttonwidgetagain–toexecuteitspick()functionagainandconfirmthatthenewseriesofnumbersdifferfromthefirstseries

Finally,restarttheapplicationandclickthe“GetMyLuckyNumbers”Buttonwidgetoncemore–andconfirmthatthisfirstseriesofnumbersaredifferenttothosenotedinthefirstserieswhentheapplicationlastran

Theseriesofgeneratednumbersarenotrepeatedeachtimetheapplicationgetslaunchedbecausetherandomgeneratorisseededbythecurrentsystemtime–whichisdifferenteachtimethegeneratorgetscalled.

DeployingapplicationsYourappsdevelopedinthePythonlanguagecanbedeployedonWindowssystemswherethePythoninterpreterisnotinstalled.Todoso,alltheprogramfilesmustbe“frozen”intoabundlethatincludesanexecutable(.exe)filetocreateasimpleMSIinstaller.The“cx_Freeze”toolisafreesetofscriptsandmodulesforfreezingPythonprogramsavailableatcx-freeze.sourceforge.net

Thecx_FreezetoolusesPython’s“distutils”packageandrequiresasetupscripttodescribeyourmoduledistributioninordertobundleappropriatesupportforyourapplication.Thesetupscriptistraditionallynamedsetup.pyandconsistsmainlyofacalltoacx_Freezesetup()function–supplyinginformationasparameterpairs.Thisspecifiesanyrequiredbuildoptions,suchasimagefilesormodulestobeincluded,andidentifiestheexecutablescriptandsystemplatformtype.Forexample,thesetupscriptfortheapplicationdevelopedthroughoutthischaptermustincludethelogoimagefilelotto.gifandspecifythefinalcodescriptnamedlotto.pyastheexecutablescript.Oncecx_Freezeisinstalled,asetupscriptcanbeexecutedfromaWindowsCommandPromptwiththeargumentbdist-msi–tocreateasub-directorynamed“dist”containingadistributableMSIinstallerforyourapp.

Thecx_FreezetoolcanalsocreateexecutablefilesforMacandLinuxsystems.Discovermoreoncx_Freezeonlineatcx_freeze.readthedocs.orgsetup.py

setup.py

InIDLEstartaPythonsetupscriptbymakingavailablethe“sys”moduleanditemsfromthe“cx_Freeze”moduleimportsys

fromcx_Freezeimportsetup,Executable

Next,addstatementstoidentifythebaseplatforminuse

base=Noneifsys.platform==‘win32’:base=‘Win32GUI’

Now,addastatementlistingoptionstobeincluded

opts={‘include_files’:[‘lotto.gif’],‘includes’:[‘re’]}

Finally,addacalltothesetup()functionpassingallinformationasparameterpairs

setup(name=‘Lotto’,

version=‘1.0’,description=‘LotteryNumberPicker’,

author=‘MikeMcGrath’,options={‘build_exe’:opts},

executables=[Executable(‘lotto.py’,base=base)])

SavethesetupscriptalongsidetheapplicationfilesthenrunthescriptcommandtocreatetheWindowsinstaller

Waituntiltheprocesscreatestheinstallerina“dist”sub-directorythencopytheinstallerontoportablemedia,suchasaUSBflashdrive

Now,copytheinstallerontoanotherWindowscomputerwherePythonmaynotbepresentandruntheinstaller

Then,selectaninstallationlocation,oracceptthesuggesteddefaultlocation

Whentheinstallerhasfinishedcopyingfiles,navigatetoyourchoseninstallationlocationandruntheexecutablefile–toseetheapplicationlaunch

Lotto-1.0-win32.msi

lotto.exe

Summary• ThestandardPythonlibraryhasarandommodulethatprovidesfunctionstogenerate

pseudo-randomnumbers

• Apseudo-randomfloating-pointnumberfrom0.0to1.0canbegeneratedbytherandommodule’srandom()function

• Multipleuniquerandomintegerswithinagivenrangecanbegeneratedbytherandommodule’ssample()function

• Aprogramplanshoulddefinetheprogram’sPurpose,requiredFunctionality,andtheInterfacewidgetsneeded

• Indesigningaprograminterfacethegrid()layoutmanagerpositionswidgetsinrowsandcolumns

• Staticpropertiesdonotchangeduringexecutionofaprogram

• Dynamicpropertiesdochangeduringexecutionofaprogramusingruntimefunctionalitytorespondtoauseraction

• Uponcompletion,aprogramshouldbetestedtoensureitperformsasexpectedineveryrespect

• Programfilescanbe“frozen”intoabundlefordistributiontoothercomputerswherethePythoninterpreterisnotpresent

• Thecx_FreezetoolusesPython’s“disutils”packagetofreezeprogramsintoexecutablesforWindows,Mac,orLinux

• Asetupscriptdescribesyourmoduledistributionsocx_Freezewillbundleappropriatesupportfortheapplication

• Traditionally,asetupscriptisnamedsetup.pyandconsistsmainlyofacalltothecx_Freezesetup()function

• ApplicationscanbedeployedonWindowssystemsusingthecx_Freezetooltocreateasimpleinstaller

• Whenasetupscriptisexecutedwiththebdist_msicommandanMSIinstalleriscreatedthatwillcopythedistributionbundleontothehostcomputer,includinganexecutablefile

13Transferringskills

Thischapterdemonstratessimilaritiesanddifferencesincodingvariouspopularprogramminglanguages.

UnderstandingcompilersCompilingcodeCodingCCodingC++CodingC#CodingJavaSummary

UnderstandingcompilersModernappsarecodedin“high-level”languagesthatprovideahighlevelofabstractionbetweenmachinecode,whichisunderstoodbycomputers,andsourcecodethatishuman-readable.Inordertorunprogramsthesourcecodemustfirstberenderedintomachinecodethatthecomputercanexecute.Thisprocessisaccomplishedeitherbyan“interpreter”orbya“compiler”dependinguponthelanguageinwhichtheprogramiswritten.

ThePythonprogramminglanguage,usedfordemonstrationthroughoutthepreviouschaptersofthisbook,usesaninterpretertotranslateprogramsourcecode.Asaprogramproceeds,theinterpreterdynamicallytranslatesitsfunctionsandstatementcodeobjectsinto“bytecode”.ThebytecodecanthenbeexecutedviaPython’sbytecodeinterpreter(a.k.a“VirtualMachine”or“VM”).

Oftenlanguagesthatemployaninterpreterarereferredtoas“interpretedlanguages”andthosethatuseacompilerarereferredtoas”compiledlanguages”.

OtherprogramminglanguagesthatemployaninterpretertotranslatesourcecodeintobytecodeforexecutionbythecomputerviatheirvirtualmachineincludeJavaandRuby.

Traditionalprogramminglanguages,suchastheClanguage,useacompilertoconvertprogramsourcecodeintomachinecode.Compilationtakestheentiresourcecodeandfirstgeneratesintermediate“objectcode”representingtheprogram’sfunctionsandstatementsinbinarymachinecodeformat.Thecompilerthencombinestheobjectcodeintoasinglebinarymachinecodefile(.exe)thatcanbeexecuteddirectlybythecomputer.

NoticethattheVirtualMachine(VM)mustbepresentontheuser’scomputertoexecutethoseprogramswrittenininterpretedlanguages.

OtherprogramminglanguagesthatemployacompilertoconvertsourcecodeintomachinecodefordirectexecutionbythecomputerincludeC++(“Cplusplus”)andC#(“Csharp”).

Thereare,therefore,fundamentaldifferencesinthewayprogramswrittenin“interpretedlanguages”,suchasPython,and“compiledlanguages”suchastheClanguage,areexecutedbythecomputer.Eachofferssomeadvantagesanddisadvantagesintermsofperformanceandportability,aslistedinthetablebelow:

Interpreter: Compiler:

Takesindividualcodeobjectsasinputfortranslation Takestheentiresourcecodeasinputforconversion

Doesnotgenerateintermediateobjectcode Doesgenerateintermediateobjectcode

Executesconditionalcontrolstatementsslowly Executesconditionalcontrolstatementsquickly

Requireslittlememoryasobjectcodeisnotgenerated Requiresmorememoryasobjectcodeisgenerated

Translatesthesourcecodeeverytimetheprogramruns Convertsthesourcecodeonceduringcompilation

Reportsanyerrorsimmediatelywhentheyoccurinastatement

Reportsanyerrorsonlyafteranattempttocompiletheentiresourcecode

Programsrunslowerwhilecodeobjectsgetindividuallytranslatedtomachinecode

Programsrunfasterwhilemachinecoderunsdirectlyonthecomputer

Distributedprogramsarehuman-readablesourcecodesoareeasytomodify

Distributedprogramsarecompiledmachinecodesoaredifficulttomodify

Offerspoorerprotectionofintellectualpropertyrights Programsrunfasterwhilemachinecoderunsdirectlyonthecomputer

Don’tbeconfusedbyJavaprograms.Theyare“compiled”intobytecodeasadistributable(.class)filethatnonethelessrequiretheJavaVMbytecodeinterpretertorun.

CompilingcodeTomorefullyunderstandthecompilationprocessitisusefultoexamineindetailthecompilationofasimpleClanguageprogram.InproducinganexecutablefilefromanoriginalCsourcecodefilethecompilationprocessactuallyundergoesfourseparatestages,whicheachgenerateanewfile:

• Preprocessing–thepreprocessorstagefirstsubstitutesall“preprocessor”directives,suchasstatementstoimportlibraries,withtheactuallibrarycode.Forinstance,intheClanguagethelibrarycodeissubstitutedfor#includeimportdirectives.Thegeneratedfilecontainingthesubstitutionsisintextformatandtypicallyhasa.ifileextension

• Translating–thecompilerstagetranslatesthehigh-levelinstructionsinthe.ifileintolow-levelAssemblylanguageinstructions.Thegeneratedfilecontainingthetranslationisintextformatandtypicallyhasa.sfileextension

• Assembling–theassemblerstageconvertstheAssemblylanguagetextinstructionsinthe.sfileintomachinecode.Thegeneratedobjectfilecontainingtheconversionisinbinaryformatandtypicallyhasa.ofileextension

• Linking–thelinkerstagecombinesoneormorebinaryobject.ofilesintoasingleexecutablefile.Thegeneratedfileisinbinaryformatandtypicallyhasa.exefileextension

Strictlyspeaking,“compilation”describesthefirstthreestagesabove,whichoperateona

singlesourcecodetextfileandultimatelygenerateasinglebinaryobjectfile.Wheretheprogramsourcecodecontainssyntaxerrors,suchasamissingstatementterminatororamissingparenthesis,theywillbereportedbythecompilerandcompilationwillfail.

Thelinker,ontheotherhand,canoperateonmultipleobjectfilesandultimatelygeneratesasingleexecutablefile.Thisallowsthecreationoflargeprogramsfrommodularobjectfilesthatmayeachcontainre-usablefunctions.Wherethelinkerfindsafunctionofthesamenamedefinedinmultipleobjectfilesitwillreportanerrorandtheexecutablefilewillnotbecreated.

Normally,thetemporaryfilescreatedduringtheintermediarystagesofthecompilationprocessareautomaticallydeleted,buttheGNUCCompiler(gcc)providesa-save-tempsoptioninthecompilercommandthatallowsthemtobesavedforinspection.

Forasimple“HelloWorld!”programintheClanguage,typeacommandgcchello.c-save-temps-ohello.exethenhitReturntocompileandsavethetemporaryfiles

Openthehello.ifileinaplaintexteditor,suchasWindows’Notepad,toseethesourcecodeappearattheveryendofthefile–precededbysubstitutedlibrarycode

Now,openthehello.sfileinaplaintexteditortoseethetranslationintolow-levelAssemblycodeandnotehowunfriendlythatappearsincontrasttohigh-levelcode

Inthecommandhere,gccinvokesthecompiler,thesourcecodefileishello.c,and-ohello.exespecifiestheexecutablefiletobeoutput.

AcompleteexampledemonstratinghowtocodeandcompileprogramsintheCprogramminglanguageisprovidedoverleaf.

CodingCThecodingdatastructuresandcontrolstructuresdescribedanddemonstratedthroughoutthisbookinthePythonprogramminglanguagealsoexistinotherprogramminglanguages.YourskillsgainedwithPythoncodingcanbetransferredtootherlanguages,suchastheCprogramminglanguage,byrecognizingsimilarities.TheCprogramminglanguageoriginatedwaybackinthe1970sandisdesignedtobeeasilycompiledtolow-levelmachinecode.Itsefficiencymakesitsuitableforawiderangeofapplicationsineverythingfromembeddedsystemstooperatingsystems.

ThesimpleGuessingGameprogram,describedinPythoncodehereandhere,canberecreatedinasimilarCequivalent.

guess.c

StartanewCprogrambyimportinglibrariestomakeinput-output,randomnumber,andtimefunctionsavailable#include<stdio.h>

#include<stdlib.h>#include<time.h>

Next,definea“main”functionbodythatwillenclosetheentiregamecodeandreturnazerovalueoncompletionintmain()

{/*Statementstobeaddedhere*/

return0;}

Now,addstatementstoinitializevariableswitharandomnumberintherange1-20,aBooleantrue,andanintegersrand(time(NULL));

intnum=(rand()%20)+1;intflag=1;

intguess=0;

Addastatementtorequestuserinput

printf(“Guessmynumber1-20:“);

Then,addaloopstatementthatreadsinputintotheintegervariableandwillencloseaconditionaltestwhile(flag==1)

{

scanf(“%d”,&guess);fflush(stdin);/*Conditionaltesttobeaddedhere*/

}

ThisCprogramwillacceptfloating-pointguessesastheirvaluegetstruncatedtoanintegerintheassignmenttotheguessvariable.

Finally,addaconditionaltestinsidetheloopthensave,compile,andruntheprogramif(guess==0){

printf(“Invalid!Enteronlydigits1-20\n”);break;

}elseif(guess<num){printf(“Toolow,tryagain:“);}

elseif(guess>num){printf(“Toohigh,tryagain:“);}else

{printf(“Correct…Mynumberis%d\n”,num);

flag=0;}

TheGNUCCompiler(gcc)iswidelyusedandisfreelyavailableaspartoftheMinimalistGNUforWindows(MinGW)packagefromsourceforge.net/projects/mingwForinstallationinstructionsandotherhelprefertothedocumentationatmingw.org/wiki

GuessingGameinC–programcomparison

• LibraryfunctionsaremadeavailableinCprogramswithan#includedirective,likeusingPython’simportkeyword

• Cprogramsalwayshaveamain()functionthatgetscalledautomaticallywhenevertheprogramruns

• StatementsinCprogramsaregroupedinside{}braces

• VariablesinCarenotlooselytypedsotheirdatatype,suchasint(integer),mustbedefinedintheirdeclaration

• TheendofeachstatementinCmustbedenotedwitha;semi-coloncharacter–tabsandspacesareirrelevant

• ControlstructuresinCprogramsuseif,else,andwhilekeywords–inmuchthesamewayasPythonprograms

CodingC++ThecodingdatastructuresandcontrolstructuresdescribedanddemonstratedthroughoutthisbookinthePythonprogramminglanguagealsoexistintheC++programminglanguage.YourskillsgainedwithPythoncodingcanbetransferredtothatlanguagebyrecognizingitssimilaritiestoPythoncode.TheC++programminglanguageoriginatedbackinthe1980sasanenhancementoftheClanguage,knownas“Cwithclasses”.TheseclassesdefineprogrammingobjectsthattransformtheproceduralnatureofCforobject-orientedprogramminginC++.

ThesimpleGuessingGameprogram,describedinPythoncodehereandhere,canberecreatedinasimilarC++equivalent.

guess.cpp

StartanewC++programbyimportinglibrariestomakeinput-output,randomnumber,andtimefunctionsavailable#include<iostream>

#include<cstdlib>#include<ctime>

Next,definea“main”functionbodythatwillenclosetheentiregamecodeandreturnazerovalueoncompletionintmain()

{/*Statementstobeaddedhere*/

return0;}

Now,addstatementstoinitializevariableswitharandomnumberintherange1-20,aBooleantrue,andanintegersrand(time(0));

intnum=(rand()%20)+1;boolflag=true;

intguess=0;

Addastatementtorequestuserinput

std::cout<<“Guessmynumber1-20:“;

Then,addaloopstatementthatreadsinputintotheintegervariableandwillencloseaconditionaltestwhile(flag==1)

{

std::cin>>guess;std::cin.ignore(256,‘\n’);/*Conditionaltesttobeaddedhere*/

}

ThisC++programwillacceptfloating-pointguessesastheirvaluegetstruncatedtoanintegerintheassignmenttotheguessvariable.

Finally,addaconditionaltestinsidetheloopthensave,compile,andruntheprogramif(guess==0){

std::cout<<“Invalid!Enteronlydigits1-20\n”;

break;}

elseif(guess<num){std::cout<<“Toolow,tryagain:“;}

elseif(guess>num){std::cout<<“Toohigh,tryagain:“;}

else{std::cout<<“Correct…Mynumberis”<<num<<“\n”;

flag=0;}

TheMinimalistGNUforWindows(MinGW)packagefromsourceforge.net/projects/mingwalsooptionallyprovidesacompilerforC++code.Inthecommandherec++invokesthecompiler,thesourcecodefileisguess.cpp,and-oguessplus.exespecifiestheexecutablefiletobeoutput.GuessingGameinC++–programcomparison

• LibraryfunctionsaremadeavailableinC++programswithan#includedirective,likeusingPython’simportkeyword

• C++programsalwayshaveamain()functionthatgetscalledautomaticallywhenevertheprogramruns

• StatementsinC++programsaregroupedinside{}braces

• VariablesinC++arenotlooselytypedsotheirdatatype,suchasint(integer),mustbedefinedintheirdeclaration

• TheendofeachstatementinC++mustbedenotedwitha;semi-coloncharacter–tabsandspacesareirrelevant

• ControlstructuresinC++programsuseif,else,andwhilekeywords–inmuchthesamewayasPythonprograms

CodingC#ThecodingdatastructuresandcontrolstructuresdescribedanddemonstratedthroughoutthisbookinthePythonprogramminglanguagealsoexistintheC#programminglanguage.YourskillsgainedwithPythoncodingcanbetransferredtothatlanguagebyrecognizingitssimilaritiestoPythoncode.TheC#programminglanguagewasdevelopedbyMicrosoftforits.NETinitiative.ProgramsinC#requiretheCommonLanguageRuntime(CLR)tobeinstalledonthehostcomputertoproducemachinecodeatruntime–aprocessknownas“Just-In-Time”(JIT)compilation.

ThesimpleGuessingGameprogram,describedinPythoncodehereandhere,canberecreatedinasimilarC#equivalent.

guess.cs

StartanewC#programbyimportingthenamespacetomakeinput-outputandrandomnumberfunctionsavailableusingSystem;

Next,defineaclassstructuretoenclosetheentiregamecodewithina“Main”functionclassGuess

{staticvoidMain()

{/*Statementstobeaddedhere*/

}};

Now,addstatementstoinitializevariableswitharandomnumberintherange1-20,aBooleantrue,andanintegerRandomgenerator=newRandom();

intnum=generator.Next(1,20+1);boolflag=true;

intguess=0;

Addastatementtorequestuserinput

Console.WriteLine(“Guessmynumber1-20:“);

Then,addaloopstatementthatreadsinputintotheintegervariableandwillencloseaconditionaltestwhile(flag==true)

{

int.TryParse(Console.ReadLine(),outguess);/*Conditionaltesttobeaddedhere*/

}

ThisC#programwillnotacceptfloating-pointguessesastheyareintelligentlyrecognizedasnon-integersbytheint.TryParse()function.

Finally,addaconditionaltestinsidetheloopthensave,compile,andruntheprogramif(guess==0){

Console.Write(“Invalid!Enteronlydigits1-20\n”);

break;}

elseif(guess<num){Console.Write(“Toolow,tryagain:“);}

elseif(guess>num){Console.Write(“Toohigh,tryagain:“);}

else{Console.Write(“Correct…Mynumberis”+num+“\n”);

flag=false;}

TheVisualStudioCommunityIDEisagreatdevelopmentenvironmentforC#programming.Itisavailablefreefromvisualstudio.comandincludestheCSharpCompiler(csc.exe).Inthecommandherecscinvokesthecompiler,/out:guesssharp.exespecifiestheexecutablefiletobeoutput,andthesourcecode

fileisnamedguess.cs.GuessingGameinC#–programcomparison

• LibraryfunctionsaremadeavailableinC#programswithausingdirective,likeusingPython’simportkeyword

• C#programsalwayshaveaclassstructureenclosingaMain()functionthatgetscalledautomaticallywhentheprogramruns

• StatementsinC#programsaregroupedinside{}braces

• VariablesinC#arenotlooselytypedsotheirdatatype,suchasbool(Boolean),mustbedefinedintheirdeclaration

• TheendofeachstatementinC#mustbedenotedwitha;semi-coloncharacter–tabsandspacesareirrelevant

• ControlstructuresinC#programsuseif,else,andwhilekeywords–inmuchthesamewayasPythonprograms

CodingJavaThecodingdatastructuresandcontrolstructuresdescribedanddemonstratedthroughoutthisbookinthePythonprogramminglanguagealsoexistintheJavaprogramminglanguage.YourskillsgainedwithPythoncodingcanbetransferredtothatlanguagebyrecognizingitssimilaritiestoPythoncode.TheJavaprogramminglanguagehastheadmirablemantra“writeonce–runanywhere”.ProgramsinJavarequiretheJavaRuntimeEnvironment(JRE)tobeinstalledonthehostcomputertoproducemachinecodeatruntime–aprocessknownas“Just-In-Time”(JIT)compilation.

ThesimpleGuessingGameprogram,describedinPythoncodehereandhere,canberecreatedinasimilarJavaequivalent.

Guess.java

StartanewJavaprogrambydefiningaclassstructuretoenclosetheentiregamecodewithina“main”functionclassGuess

{publicstaticvoidmain(String[]args)

{/*Statementstobeaddedhere*/

}};

Now,addstatementstoinitializevariableswitharandomnumberintherange1-20,aBooleantrue,andanintegerintnum=(int)(Math.random()*20+1);

booleanflag=true;intguess=0;

Addastatementtorequestuserinput

System.out.print(“Guessmynumber1-20:“);

Then,addaloopstatementthatreadsinputintotheintegervariableandwillencloseaconditionaltestwhile(flag==true)

{try{

guess=Integer.parseInt(System.console().readLine());}catch(NumberFormatExceptionex){}

/*Conditionaltesttobeaddedhere*/

}

ThisJavaprogramwillnotacceptfloating-pointguessesastheyarerecognizedasnon-integersbythefunctionInteger.parseInt().

Finally,addaconditionaltestinsidetheloopthensave,compile,andruntheprogramif(guess==0){

System.out.println(“Invalid!Enteronlydigits1-20”);break;

}elseif(guess<num){

System.out.print(“Toolow,tryagain:“);}elseif(guess>num){

System.out.print(“Toohigh,tryagain:“);}else{

System.out.println(“Correct…Mynumberis”+num);flag=false;

}

TheJavaDevelopmentKit(JDK)isneededtocreateJavaprograms.Itisavailablefreefromoracle.comandincludesthecommand-lineJavaCompiler(javac.exe).InthecommandherejavacinvokesthecompilerforGuess.java

sourcecodeandwillautomaticallyoutputanexecutablefilenamedGuess.exe.ThiscanthenberunbytheJREusingthejavacommand.GuessingGameinJava–programcomparison

• Input-outputandrandomnumberfunctionsarereadilyavailableinJavaprogramswithoutimportstatements

• Javaprogramsalwayshaveaclassstructureenclosingamain()functionthatgetscalledautomaticallywhentheprogramruns

• StatementsinJavaprogramsaregroupedinside{}braces

• VariablesinJavaarenotlooselytypedsotheirdatatype,suchasboolean,mustbedefinedintheirdeclaration

• TheendofeachstatementinJavamustbedenotedwitha;semi-coloncharacter–tabsandspacesareirrelevant

• ControlstructuresinJavaprogramsuseif,else,andwhilekeywords–inmuchthesamewayasPythonprograms

Summary• Modernprogramminglanguages,likePythonorC,provideahighlevelofabstraction

fromlowlevelmachinecode

• Human-readablehigh-levelsourcecodecanberenderedintolow-levelmachinecodebyaninterpreterorbyacompiler

• Interpretedprogramminglanguages,suchasPython,translatesourcecodeintobytecode,whichcanthenbeexecutedviatheirVirtualMachinebytecodeinterpreter

• Compiledprogramminglanguages,suchasC,generateintermediateobjectcodethatgetscombinedintomachinecode,whichcanthenbeexecuteddirectlyonthecomputer

• ThecompilationofaCprogramtranslateshigh-levelsourcecodeintolow-levelAssemblylanguagethenmachinecode

• Intermediatefilesgeneratedduringthecompilationprocessarenormallydeletedautomaticallybythecompiler

• ThedatastructuresandcontrolstructuresusedinPythonalsoexistintheC,C++,C#,andJavaprogramminglanguages–soprogrammingskillsaretransferrableacrosslanguages

• Compiledprogramminglanguages,suchasC,haveamainfunctionthatiscalledautomaticallywhentheprogramruns

• Statementsinmanyprogramminglanguagesaregroupedwithin{}bracesandmustendwitha;semi-colon

• Programminglanguagesthathavestronglytypedvariablesrequirethedatatypethatthevariablemaycontain,suchasintinteger,tobedefinedinthevariabledeclaration

• LikePython,otherprogramminglanguagescanalsoimportfunctionalityfromtheirlibraries

• C#programsrequiretheCommonLanguageRuntime(CLR)tobeinstalledtoproducemachinecodeatruntime

• JavaprogramsrequiretheJavaRuntimeEnvironment(JRE)tobeinstalledtoproducemachinecodeatruntime