prepared by joseph ghafari, mandy bitar. introduction & history python was conceived in the late...

Prepared by JOSEPH GHAFARI, MANDY BITAR

Introduction & History Python was conceived in the late 1980s by Guido van Rossum at the National Research Institute for Mathematics and Computer Science in the Netherlands as a successor of the ABC programming language and first published and released in 1991. Python supports multiple programming paradigms (primarily functional, object oriented and imperative), and features a fully dynamic type system and automatic memory management; it is thus similar to Perl, Ruby, Scheme. To try Python just install the Python interpreter and we can use the IDLE (Integrated Development Environment) that comes with the interpreter. Python makes it easy to run single lines of code (its what we call one-liner programs). To make bigger programs in Python you just have to write the whole program in a notepad and then save it as *.py and open the file with the Python IDLE and then run the program. NASA uses Python.

Very Simple Programs We start by creating simple programs with single lines of code: To output something on the screen we type print . Commas outside the quotations serve as a concatenation when used with the print operator. Math in Python: Python Operators: In the division if we type 19.0/3.0 the output will be 6.33333 because in Python unlike JAVA we dont have float double etc Order of Operations: parentheses () ; exponents ** ; multiplication *,division \ and remainder % ; addition + and subtraction -.

Simple Programs (Cont) Math operations work only outside the quotations in a print statement concatenated using commas. A comment is a piece of code that is not run. In python, you make something a comment by putting a hash # in front of it. A hash comments everything after it in the line, and nothing before it. VARIABLES : Variables store a value just like java but the difference is that in Python we dont declare a type for the variable. A variable can be an integer a string etc Its very easy to declare a variable in Python for example : v = 1 or v = v * 5 or v = Hello etc

The While Loop The While Loop works just like the while in java but without the use of curly braces nor the parentheses in the while statement. So the code that we want to loop must be indented after the while statement or else it wont be part of the loop. For example a while loop in Python would be in this form: while condition that the loop continues: what to do in the loop have it indented the code here is not looped because it isn't indented

Boolean Expressions Boolean expressions are used to set conditions for the loop to continue. Basically a loop keeps going if the condition is true, otherwise the loop stops. Boolean Operators:

Conditional Operators Unlike loops, conditionals are only run once. The most common conditional in any programming language, is the if statement. There is also the else statement and the elif statement which is the same as elseif in JAVA. Here is how its written in Python: if conditions: run this code# similar to the while loop the code inside elif conditions: # the if elif and else must be INDENTED run this code elif conditions: run this code else: run this code but this happens regardless because it isn't indented

Functions Functions are little self-contained programs that perform a specific task, which can be used at any time, in any place and can be incorporated into larger programs. Python has lots of pre-made functions that can be used right now by simply calling them. Here is the general form that calling a function takes: function_name(parameters) Suppose a function called user_input that asks the user to type in something then turns it into a string of text for example: a = user_input("Type in something, and it will be printed on screen:") print a The return value of the function will be stored in the variable a.

Functions (Cont) To define our own Functions we use the def operator: def function_name (parameter_1,parameter_2): this is the code in the function more code return value to return to the main program this code isn't in the function because it isn't indented The only thing functions see of the main program is the parameters that are passed to it. The only thing the main program sees of functions is the returned value that it passes back.

Tuples A Tuple is a list of values, but you cant change their values once you initialize it. Each value is numbered starting from zero. For example: the names of the months of the year: months = ('January','February','March','April','May','June',\ 'July','August','September','October','November',' December') print months[2]# printing the 3 rd month, march. print months# print the whole list. The \ at the end of the first line carries over that line of code to the next line. The parentheses are optional it just stops Python from getting things confused.

Lists Lists are a list of values. Each one of them is numbered, starting from zero. You can remove values from the list, and add new values to the end. For example: Your cats names. cats = ['Tom', 'Snappy', 'Kitty', 'Jessie', 'Chester'] print cats[2] # printing the name of the 3 rd cat, Kitty. cats[0:2] # would recall the1st and 2nd cats.(range) cats.sort()# sorts the list alphabetically. The code is the same as a tuple, EXCEPT that all the values are put between square brackets, not parentheses. To add a value to the list, just use the append() function: cats.append('Catherine') # adds Catherine to the end of the list. To delete a value from the list, just use the del operator: del cats[1] # removes the 2nd cat, Snappy.

Dictionaries In a dictionary, you have an index of words, and their definitions. In Python, the word is called a key, and the definition a value. For example a telephone book where you give the computer a name, and it gives us a number. Name = Key, Number = Value. phonebook = {'Andrew Parson':8806336, \ 'Emily Everett':6784346, 'Peter Power':7658344, \ 'Lewis Lame':1122345} # we use curly braces instead. print phonebook['Lewis Lame']# print his number on screen. To add a person to the phonebook just type: phonebook['Gingerbread Man'] = 1234567 To delete a person from the phonebook, just use the del operator: del phonebook['Andrew Parson']# deletes Andrew from the phonebook

Dictionaries (Cont) Using the function has_key(): function_name.has_key(key-name)#It returns True if the dictionary #has key-name in it, False otherwise. Using the function keys(): phonebook.keys()#This returns a list of all the names of the keys. You could use this function to put all the key names in a list: keys = phonebook.keys() Using the function values(): phonebook.values()#This returns a list of all the values in a phonebook. You can't sort dictionaries because they are in no particular order. You can find the number of entries with the len() function: print "The phonebook has", \ len(phonebook), "entries in it"

The For Loop The for loop does something for every value in a list. For example: # First, create a list to loop through: newList = [45, 'eat me', 90210, "The day has come, the walrus said, \ to speak of many things", -67] # Second, create the loop: # Goes through newList, and sequentially puts each bit of information # into the variable value, and runs the loop: for value in newList: print value As you see, when the loop executes, it runs through all of the values in the list mentioned after 'in'. It then puts them into the variable value, and executes through the loop, each time with value being worth something different.

Classes Since Python is an object-oriented-programming language we can create classes that relates functions and variables together in a way that they can see each other and work together. We create a class using the class operator, The function __init__ is run when we create an instance of a class. (constructor in JAVA) The operator self is how we refer to things in the class from within itself. It is the first parameter in any function defined inside a class. Any function or variable created on the first level of indentation of where we put class Shape is automatically put into self. To access these functions and variables elsewhere inside the class, their name must be preceded with self and the dot . operator. (e.g. self.variable_name). For example the definition of a shape:

Classes (Cont) class Shape: def __init__(self,x,y): self.x = x self.y = y description = "This shape has not been described yet author = "Nobody has claimed to make this shape yet" def area(self): return self.x * self.y def perimeter(self): return 2 * self.x + 2 * self.y def describe(self,text): self.description = text def authorName(self,text): self.author = text def scaleSize(self,scale): self.x = self.x * scale self.y = self.y * scale

Classes (Cont) Using a class: rectangle = Shape(100,45) #creating an instance of a class. print rectangle.area() #finding the area of the rectangle. print rectangle.perimeter() #finding the perimeter of the rectangle. rectangle.describe("A wide rectangle, more than twice\ as wide as it is tall") #describing the rectangle. rectangle.scaleSize(0.5) #making the rectangle 50% smaller. longrectangle = Shape(120,10) #another instance of shape. fatrectangle = Shape(130,120) #both have their own functions and #variables contained inside them.

Inheritance We define a new class, based on another, parent class. Our new class brings everything over from the parent, and we can also add other things to it. If any new attributes or methods have the same name as an attribute or method in our parent class, it is used instead of the parent one. Using inheritance we can easily define a Square: class Square(Shape): def __init__(self,x): self.x = x self.y = x We redefined the __init__ function of Shape so that the X and Y values are the same. (we have overridden the constructor)

Pointers & Dictionaries of Classes Consider the following statement in Python: instance2 = instance1 #instance2 is pointing to instance1 There are two names given to the one class instance, and you can access the class instance via either name. In other languages, you do things like this using pointers(C++), however in python this all happens behind the scenes just like in JAVA. In addition we can assign an instance of a class to an entry in a list or dictionary. # First, create a dictionary: dictionary = {} # Then, create some instances of classes in the dictionary: dictionary["long rectangle"] = Shape(600,45) #You can now use them like a normal class: print dictionary["long rectangle"].area()

Modules A module (packages in JAVA) is a Python file that has only definitions of variables, functions, and classes. We import bits of it (or all of it) into other programs using the import operator. For example, to import moduletest.py into your main program we write: import moduletest This assumes that the module is in the same directory as mainprogram.py, or is a default module that comes with Python. You normally put all import statements at the beginning of the Python file, but technically they can be anywhere. In order to use the items in the module in your main program, use the following: # modulename.itemname print moduletest.ageofqueen piano = moduletest.Piano()

Modules (Cont) One way to get rid of the modulename is to import only the wanted objects from the module. To do this, you use the from operator: # from modulename import itemname from moduletest import printhello printhello() We can also import a module in the normal way (without the from operator) and then assign items to a local name: timesfour = moduletest.timesfour print timesfour(565)

File Input/Output To open a text file you use the open() function. You pass certain parameters to it to tell it in which way the file should be opened: r for read only, w for writing only, a for appending and r+ for both reading and writing. Example: openfile = open('pathtofile', 'r') openfile.read() print openfile.read() wont work because the cursor has changed its place. This cursor tells the read function where to start from. To set the cursor, you use the seek() function. It is used in the form: seek(offset, whence) whence is optional, and determines where to start from. If whence is 0, the bytes/letters are counted from the beginning. If it is 1, the bytes are counted from the current cursor position. If it is 2, then the bytes are counted from the end of the file. If nothing is put there, 0 is assumed. offset decribes how far from whence the cursor moves.

File Input/Output (Cont) For example: openfile.seek(45,0) #moves cursor 45 bytes/letters after the beginning of the file. openfile.seek(10,1) #moves cursor 10 b/letters after the current cursor position. openfile.seek(-77,2) #moves cursor 77 bytes/letters before the end of the file. print openfile.read() #It will print from the spot you seeked to. After this is done #the cursor will be at the end of the file so to print #something again you will have to seek again. More I/O Functions: openfile.tell() #takes no parameters/returns where the cursor is in the file. openfile.readline() #takes no parameters/reads from where the cursor is till the #end of the line. You can optionally pass to it the maximum number of bytes/letters to read by putting a number in the brackets.

File Input/Output (Cont) openfile.readlines() # is much like readline(), however readlines() reads all the lines from the cursor onwards, and returns a list, with each list element holding a line of code. For example: If you had the text file:The returned list would be: openfile.write('this is a string') # It writes from where the cursor is, and overwrites text in front of it. openfile.close() # It closes the file so that you can no longer read or write to it until you reopen it again.

') except (NameError,SyntaxError): loop = 0">

Exception Handling print 'Subtraction program:' #to handle exceptions we use the try and except operators. loop = 1#except without parameters catches all errors. while loop == 1: try: a = input('Enter a number to subtract from > ') b = input('Enter the number to subtract > ') except NameError: print "\nYou cannot subtract a letter" except SyntaxError: print "\nPlease enter a number only." print a - b try: loop = input('Press 1 to try again > ') except (NameError,SyntaxError): loop = 0

prepared by joseph ghafari, mandy bitar. introduction & history python was conceived in the late...

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