python programming: an introduction to computer...
TRANSCRIPT
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Python Programming, 3/e 1
Python Programming: An Introduction to Computer Science
Chapter 5 Sequences: Strings, Lists, and Files
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Python Programming, 3/e 2
Objectives n To understand the string data type and
how strings are represented in the computer.
n To become familiar with various operations that can be performed on strings through built-in functions and string methods.
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Python Programming, 3/e 3
Objectives n To understand the basic idea of sequences
and indexing as they apply to Python strings and lists.
n To be able to apply string formatting to produce attractive, informative program output.
n To understand basic file processing concepts and techniques for reading and writing text files in Python.
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Python Programming, 3/e 4
Objectives n To understand basic concepts of
cryptography. n To be able to understand and write
programs that process textual information.
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Python Programming, 3/e 5
The String Data Type n The most common use of personal
computers is word processing. n Text is represented in programs by the
string data type. n A string is a sequence of characters
enclosed within quotation marks (") or apostrophes (').
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Python Programming, 3/e 6
The String Data Type >>> str1="Hello" >>> str2='spam'
>>> print(str1, str2) Hello spam
>>> type(str1)
<class 'str'> >>> type(str2)
<class 'str'>
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Python Programming, 3/e 7
The String Data Type n Getting a string as input
>>> firstName = input("Please enter your name: ")
Please enter your name: John
>>> print("Hello", firstName)
Hello John
n Notice that the input is not evaluated. We want to store the typed characters, not to evaluate them as a Python expression.
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Python Programming, 3/e 8
The String Data Type n We can access the individual characters
in a string through indexing. n The positions in a string are numbered
from the left, starting with 0. n The general form is <string>[<expr>],
where the value of expr determines which character is selected from the string.
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Python Programming, 3/e 9
The String Data Type
>>> greet = "Hello Bob" >>> greet[0]
'H'
>>> print(greet[0], greet[2], greet[4])
H l o
>>> x = 8
>>> print(greet[x - 2])
B
H e l l o B o b
0 1 2 3 4 5 6 7 8
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Python Programming, 3/e 10
The String Data Type
n In a string of n characters, the last character is at position n-1 since we start counting with 0.
n We can index from the right side using negative indexes.
>>> greet[-1] 'b' >>> greet[-3] 'B'
H e l l o B o b
0 1 2 3 4 5 6 7 8
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Python Programming, 3/e 11
The String Data Type n Indexing returns a string containing a
single character from a larger string. n We can also access a contiguous
sequence of characters, called a substring, through a process called slicing.
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Python Programming, 3/e 12
The String Data Type n Slicing: <string>[<start>:<end>]
n start and end should both be ints n The slice contains the substring
beginning at position start and runs up to but doesn’t include the position end.
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Python Programming, 3/e 13
The String Data Type
>>> greet[0:3] 'Hel' >>> greet[5:9] ' Bob' >>> greet[:5] 'Hello' >>> greet[5:] ' Bob' >>> greet[:] 'Hello Bob'
H e l l o B o b
0 1 2 3 4 5 6 7 8
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Python Programming, 3/e 14
The String Data Type n If either expression is missing, then the
start or the end of the string are used. n Can we put two strings together into a
longer string? n Concatenation “glues” two strings
together (+) n Repetition builds up a string by multiple
concatenations of a string with itself (*)
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Python Programming, 3/e 15
The String Data Type n The function len will return the length of a
string. >>> "spam" + "eggs" 'spameggs' >>> "Spam" + "And" + "Eggs" 'SpamAndEggs' >>> 3 * "spam" 'spamspamspam' >>> "spam" * 5 'spamspamspamspamspam' >>> (3 * "spam") + ("eggs" * 5) 'spamspamspameggseggseggseggseggs'
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Python Programming, 3/e 16
The String Data Type >>> len("spam")
4 >>> for ch in "Spam!":
print (ch, end=" ")
S p a m !
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Python Programming, 3/e 17
The String Data Type
Operator Meaning
+ Concatenation
* Repetition
<string>[] Indexing
<string>[:] Slicing
len(<string>) Length for <var> in <string> Iteration through characters
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Python Programming, 3/e 18
Simple String Processing
n Usernames on a computer system n First initial, first seven characters of last name
# get user’s first and last names
first = input("Please enter your first name (all lowercase): ")
last = input("Please enter your last name (all lowercase): ")
# concatenate first initial with 7 chars of last name
uname = first[0] + last[:7]
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Python Programming, 3/e 19
Simple String Processing >>>
Please enter your first name (all lowercase): john Please enter your last name (all lowercase): doe
uname = jdoe
>>>
Please enter your first name (all lowercase): donna Please enter your last name (all lowercase): rostenkowski
uname = drostenk
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Python Programming, 3/e 20
Simple String Processing n Another use – converting an int that
stands for the month into the three letter abbreviation for that month.
n Store all the names in one big string: “JanFebMarAprMayJunJulAugSepOctNovDec”
n Use the month number as an index for slicing this string: monthAbbrev = months[pos:pos+3]
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Python Programming, 3/e 21
Simple String Processing
Month Number Position
Jan 1 0
Feb 2 3
Mar 3 6
Apr 4 9
§ To get the correct position, subtract one from the month number and multiply by three
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Python Programming, 3/e 22
Simple String Processing # month.py # A program to print the abbreviation of a month, given its number
def main(): # months is used as a lookup table months = "JanFebMarAprMayJunJulAugSepOctNovDec"
n = int(input("Enter a month number (1-12): "))
# compute starting position of month n in months pos = (n-1) * 3
# Grab the appropriate slice from months monthAbbrev = months[pos:pos+3]
# print the result print ("The month abbreviation is", monthAbbrev + ".")
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Python Programming, 3/e 23
Simple String Processing >>> main() Enter a month number (1-12): 1
The month abbreviation is Jan.
>>> main()
Enter a month number (1-12): 12
The month abbreviation is Dec.
n One weakness – this method only works where the potential outputs all have the same length.
n How could you handle spelling out the months?
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Python Programming, 3/e 24
Lists as Sequences n It turns out that strings are really a special
kind of sequence, so these operations also apply to sequences!
>>> [1,2] + [3,4] [1, 2, 3, 4] >>> [1,2]*3 [1, 2, 1, 2, 1, 2] >>> grades = ['A', 'B', 'C', 'D', 'F'] >>> grades[0] 'A' >>> grades[2:4] ['C', 'D'] >>> len(grades) 5
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Python Programming, 3/e 25
Lists as Sequences n Strings are always sequences of
characters, but lists can be sequences of arbitrary values.
n Lists can have numbers, strings, or both! myList = [1, "Spam ", 4, "U"]
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Python Programming, 3/e 26
Lists as Sequences n We can use the idea of a list to make
our previous month program even simpler!
n We change the lookup table for months to a list: months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
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Python Programming, 3/e 27
Lists as Sequences n To get the months out of the sequence,
do this: monthAbbrev = months[n-1] Rather than this: monthAbbrev = months[pos:pos+3]
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Python Programming, 3/e 28
Lists as Sequences # month2.py # A program to print the month name, given it's number. # This version uses a list as a lookup table. def main(): # months is a list used as a lookup table months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"] n = int(input("Enter a month number (1-12): ")) print ("The month abbreviation is", months[n-1] + ".")
n Note that the months line overlaps a line. Python knows that the expression isn’t complete until the closing ‘]’ is encountered.
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Python Programming, 3/e 29
Lists as Sequences # month2.py # A program to print the month name, given it's number. # This version uses a list as a lookup table.
def main():
# months is a list used as a lookup table months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
n = int(input("Enter a month number (1-12): "))
print ("The month abbreviation is", months[n-1] + ".")
n Since the list is indexed starting from 0, the n-1 calculation is straight-forward enough to put in the print statement without needing a separate step.
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Python Programming, 3/e 30
Lists as Sequences n This version of the program is easy to
extend to print out the whole month name rather than an abbreviation! months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
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Python Programming, 3/e 31
Lists as Sequences n Lists are mutable, meaning they can be
changed. Strings can not be changed. >>> myList = [34, 26, 15, 10] >>> myList[2] 15 >>> myList[2] = 0 >>> myList [34, 26, 0, 10] >>> myString = "Hello World" >>> myString[2] 'l' >>> myString[2] = "p" Traceback (most recent call last): File "<pyshell#16>", line 1, in -toplevel- myString[2] = "p" TypeError: object doesn't support item assignment
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Python Programming, 3/e 32
String Representation n Inside the computer, strings are
represented as sequences of 1’s and 0’s, just like numbers.
n A string is stored as a sequence of binary numbers, one number per character.
n It doesn’t matter what value is assigned as long as it’s done consistently.
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Python Programming, 3/e 33
String Representation n In the early days of computers, each
manufacturer used their own encoding of numbers for characters.
n ASCII system (American Standard Code for Information Interchange) uses 127 bit codes
n Python supports Unicode (100,000+ characters)
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Python Programming, 3/e 34
String Representation n The ord function returns the numeric
(ordinal) code of a single character. n The chr function converts a numeric code to
the corresponding character. >>> ord("A") 65 >>> ord("a") 97 >>> chr(97) 'a' >>> chr(65) 'A'
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Python Programming, 3/e 35
Programming an Encoder n Using ord and char we can convert a string
into and out of numeric form. n The encoding algorithm is simple: get the message to encode for each character in the message: print the letter number of the character
n A for loop iterates over a sequence of objects, so the for loop looks like: for ch in <string>
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Python Programming, 3/e 36
Programming an Encoder # text2numbers.py # A program to convert a textual message into a sequence of # numbers, utlilizing the underlying Unicode encoding. def main(): print("This program converts a textual message into a sequence") print ("of numbers representing the Unicode encoding of the message.
\n") # Get the message to encode message = input("Please enter the message to encode: ") print("\nHere are the Unicode codes:") # Loop through the message and print out the Unicode values for ch in message: print(ord(ch), end=" ") print() # blank line before prompt
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Python Programming, 3/e 37
Programming a Decoder n We now have a program to convert
messages into a type of “code”, but it would be nice to have a program that could decode the message!
n The outline for a decoder: get the sequence of numbers to decode message = “” for each number in the input: convert the number to the appropriate character add the character to the end of the message print the message
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Python Programming, 3/e 38
Programming a Decoder n The variable message is an accumulator
variable, initially set to the empty string, the string with no characters ("").
n Each time through the loop, a number from the input is converted to the appropriate character and appended to the end of the accumulator.
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Python Programming, 3/e 39
Programming a Decoder n How do we get the sequence of
numbers to decode? n Read the input as a single string, then
split it apart into substrings, each of which represents one number.
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Python Programming, 3/e 40
Programming a Decoder
n The new algorithm get the sequence of numbers as a string, inString split inString into a sequence of smaller strings message = "" for each of the smaller strings: change the string of digits into the number it represents append the ASCII character for that number to message print message
n Strings are objects and have useful methods associated with them
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Python Programming, 3/e 41
Programming a Decoder n One of these methods is split. This will
split a string into substrings based on spaces.
>>> "Hello string methods!".split() ['Hello', 'string', 'methods!']
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Python Programming, 3/e 42
Programming a Decoder n Split can be used on characters other
than space, by supplying the character as a parameter.
>>> "32,24,25,57".split(",") ['32', '24', '25', '57']
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Python Programming, 3/e 43
Programming a Decoder
n We could get the x and y values of a point in a single input string by… n Turning it into a list using the split method n Indexing the individual component strings n Convert these strings into their corresponding
numbers using int or float coords = input("Enter the point coordinates (x,y): ").split(",") x,y = float(coords[0]), float(coords[1])
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Python Programming, 3/e 44
Programming a Decoder # numbers2text.py # A program to convert a sequence of Unicode numbers into # a string of text.
def main(): print ("This program converts a sequence of Unicode numbers into") print ("the string of text that it represents.\n")
# Get the message to encode inString = input("Please enter the Unicode-encoded message: ")
# Loop through each substring and build Unicde message message = "" for numStr in inString.split(i): # convert the (sub)string to a number codeNum = int(numStr) # append character to message message = message + chr(codeNum)
print("\nThe decoded message is:", message)
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Python Programming, 3/e 45
Programming a Decoder n The split function produces a
sequence of strings. numString gets each successive substring.
n Each time through the loop, the next substring is converted to the appropriate Unicode character and appended to the end of message.
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Python Programming, 3/e 46
Programming a Decoder ------------------------------------------------------------------------- This program converts a textual message into a sequence
of numbers representing the Unicode encoding of the message.
Please enter the message to encode: CS120 is fun!
Here are the Unicode codes:
67 83 49 50 48 32 105 115 32 102 117 110 33
--------------------------------------------------------------------------
This program converts a sequence of Unicode numbers into
the string of text that it represents.
Please enter the ASCII-encoded message: 67 83 49 50 48 32 105 115 32 102 117 110 33
The decoded message is: CS120 is fun!
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More String Methods n There are a number of other string
methods. Try them all! n s.capitalize() – Copy of s with only the
first character capitalized n s.title() – Copy of s; first character of
each word capitalized n s.center(width) – Center s in a field of
given width
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Python Programming, 3/e 48
More String Methods n s.count(sub) – Count the number of
occurrences of sub in s n s.find(sub) – Find the first position
where sub occurs in s n s.join(list) – Concatenate list of
strings into one large string using s as separator.
n s.ljust(width) – Like center, but s is left-justified
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Python Programming, 3/e 49
More String Methods n s.lower() – Copy of s in all lowercase
letters n s.lstrip() – Copy of s with leading
whitespace removed n s.replace(oldsub, newsub) – Replace
occurrences of oldsub in s with newsub n s.rfind(sub) – Like find, but returns the
right-most position n s.rjust(width) – Like center, but s is
right-justified
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Python Programming, 3/e 50
More String Methods n s.rstrip() – Copy of s with trailing
whitespace removed n s.split() – Split s into a list of
substrings n s.upper() – Copy of s; all characters
converted to uppercase
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Python Programming, 3/e 51
Lists Have Methods, Too
n The append method can be used to add an item at the end of a list.
squares = [] for x in range(1,101): squares.append(x*x)
n We start with an empty list ([]) and each number from 1 to 100 is squared and appended to it ([1, 4, 9, …, 10000]).
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Python Programming, 3/e 52
Lists Have Methods, Too n We can use an alternative approach in
the decoder program. n The statement message = message + chr(codeNum) essentially creates a copy of the message so far and tacks one character on the end.
n As we build up the message, we keep recopying a longer and longer string just to add a single character at the end!
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Python Programming, 3/e 53
Lists Have Methods, Too n We can avoid this recopying by using
lists of characters where each new character is appended to the end of the existing list.
n Since lists are mutable, the list is changed “in place” without having to copy the content over to a new object.
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Python Programming, 3/e 54
Lists Have Methods, Too n When done, we can use join to
concatenate the characters into a string.
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Python Programming, 3/e 55
Lists Have Methods, Too # numbers2text2.py # A program to convert a sequence of Unicode numbers into # a string of text. Efficient version using a list accumulator.
def main(): print("This program converts a sequence of Unicode numbers into") print("the string of text that it represents.\n")
# Get the message to encode inString = input("Please enter the Unicode-encoded message: ")
# Loop through each substring and build Unicode message chars = [] for numStr in inString.split():
codeNum = int(numStr) # convert digits to a number chars.append(chr(codeNum)) # accumulate new character message = "".join(chars)
print("\nThe decoded message is:", message)
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From Encoding to Encryption n The process of encoding information for the
purpose of keeping it secret or transmitting it privately is called encryption.
n Cryptography is the study of encryption methods.
n Encryption is used when transmitting credit card and other personal information to a web site.
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Python Programming, 3/e 57
From Encoding to Encryption n Strings are represented as a sort of
encoding problem, where each character in the string is represented as a number that’s stored in the computer.
n The code that is the mapping between character and number is an industry standard, so it’s not “secret”.
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From Encoding to Encryption n The encoding/decoding programs we
wrote use a substitution cipher, where each character of the original message, known as the plaintext, is replaced by a corresponding symbol in the cipher alphabet.
n The resulting code is known as the ciphertext.
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From Encoding to Encryption n This type of code is relatively easy to
break. n Each letter is always encoded with the
same symbol, so using statistical analysis on the frequency of the letters and trial and error, the original message can be determined.
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From Encoding to Encryption n Modern encryption converts messages
into numbers. n Sophisticated mathematical formulas
convert these numbers into new numbers – usually this transformation consists of combining the message with another value called the “key”
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Python Programming, 3/e 61
From Encoding to Encryption n To decrypt the message, the receiving end
needs an appropriate key so the encoding can be reversed.
n In a private key system the same key is used for encrypting and decrypting messages. Everyone you know would need a copy of this key to communicate with you, but it needs to be kept a secret.
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Python Programming, 3/e 62
From Encoding to Encryption n In public key encryption, there are separate
keys for encrypting and decrypting the message.
n In public key systems, the encryption key is made publicly available, while the decryption key is kept private.
n Anyone with the public key can send a message, but only the person who holds the private key (decryption key) can decrypt it.
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Input/Output as String Manipulation
n Often we will need to do some string operations to prepare our string data for output (“pretty it up”)
n Let’s say we want to enter a date in the format “05/24/2015” and output “May 24, 2015.” How could we do that?
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Python Programming, 3/e 64
Input/Output as String Manipulation
Input the date in mm/dd/yyyy format (dateStr)
Split dateStr into month, day, and year strings Convert the month string into a month number
Use the month number to lookup the month name
Create a new date string in the form “Month Day, Year”
Output the new date string
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Input/Output as String Manipulation
n The first two lines are easily implemented! dateStr = input("Enter a date (mm/dd/yyyy): ") monthStr, dayStr, yearStr = dateStr.split("/")
n The date is input as a string, and then “unpacked” into the three variables by splitting it at the slashes and using simultaneous assignment.
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Input/Output as String Manipulation
n Next step: Convert monthStr into a number
n We can use the int function on monthStr to convert "05", for example, into the integer 5. (int("05") = 5)
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Python Programming, 3/e 67
Input/Output as String Manipulation
n Note: eval would work, but for the leading 0 >>> int("05") 5 >>> eval("05") Traceback (most recent call last): File "<pyshell#9>", line 1, in <module> eval("05") File "<string>", line 1 05
^ SyntaxError: invalid token
n This is historical baggage. A leading 0 used to be used for base 8 (octal) literals in Python.
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Input/Output as String Manipulation months = ["January", "February", …, "December"]
monthStr = months[int(monthStr) – 1]
n Remember that since we start counting at 0, we need to subtract one from the month.
n Now let’s concatenate the output string together!
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Input/Output as String Manipulation
print ("The converted date is:", monthStr, dayStr+",", yearStr)
n Notice how the comma is appended to dayStr with concatenation!
n >>> main() Enter a date (mm/dd/yyyy): 01/23/2010 The converted date is: January 23, 2010
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Input/Output as String Manipulation n Sometimes we want to convert a number into
a string. n We can use the str function. >>> str(500) '500' >>> value = 3.14 >>> str(value) '3.14' >>> print("The value is", str(value) + ".") The value is 3.14.
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Python Programming, 3/e 71
Input/Output as String Manipulation
n If value is a string, we can concatenate a period onto the end of it.
n If value is an int, what happens? >>> value = 3.14 >>> print("The value is", value + ".") The value is Traceback (most recent call last): File "<pyshell#10>", line 1, in -toplevel- print "The value is", value + "." TypeError: unsupported operand type(s) for +: 'float' and
'str'
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Python Programming, 3/e 72
Input/Output as String Manipulation
n We now have a complete set of type conversion operations: Function Meaning
float(<expr>) Convert expr to a floating point value
int(<expr>) Convert expr to an integer value
str(<expr>) Return a string representation of expr
eval(<string>) Evaluate string as an expression
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Python Programming, 3/e 73
String Formatting n String formatting is an easy way to get
beautiful output! Change Counter
Please enter the count of each coin type.
Quarters: 6
Dimes: 0
Nickels: 0
Pennies: 0
The total value of your change is 1.5
n Shouldn’t that be more like $1.50??
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Python Programming, 3/e 74
String Formatting
n We can format our output by modifying the print statement as follows: print("The total value of your change is ${0:0.2f}".format(total))
n Now we get something like:
The total value of your change is $1.50 n Key is the string format method.
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String Formatting n <template-string>.format(<values>)
n {} within the template-string mark “slots” into which the values are inserted.
n Each slot has description that includes format specifier telling Python how the value for the slot should appear.
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Python Programming, 3/e 76
String Formatting print("The total value of your change is ${0:0.2f}".format(total)
n The template contains a single slot with the description: 0:0.2f
n Form of description: <index>:<format-specifier>
n Index tells which parameter to insert into the slot. In this case, total.
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Python Programming, 3/e 77
String Formatting n The formatting specifier has the form: <width>.<precision><type>
n f means "fixed point" number n <width> tells us how many spaces to
use to display the value. 0 means to use as much space as necessary.
n <precision> is the number of decimal places.
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Python Programming, 3/e 78
String Formatting >>> "Hello {0} {1}, you may have won ${2}" .format("Mr.", "Smith", 10000) 'Hello Mr. Smith, you may have won $10000'
>>> 'This int, {0:5}, was placed in a field of width 5'.format(7) 'This int, 7, was placed in a field of width 5'
>>> 'This int, {0:10}, was placed in a field of witdh 10'.format(10) 'This int, 10, was placed in a field of witdh 10'
>>> 'This float, {0:10.5}, has width 10 and precision 5.'.format(3.1415926) 'This float, 3.1416, has width 10 and precision 5.'
>>> 'This float, {0:10.5f}, is fixed at 5 decimal places.'.format(3.1415926) 'This float, 3.14159, has width 0 and precision 5.'
>>> "Compare {0} and {0:0.20}".format(3.14) 'Compare 3.14 and 3.1400000000000001243'
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String Formatting n Numeric values are right-justified and strings
are left- justified, by default. n You can also specify a justification before the
width. >>> "left justification: {0:<5}.format("Hi!")
'left justification: Hi! ' >>> "right justification: {0:>5}.format("Hi!")
'right justification: Hi!'
>>> "centered: {0:^5}".format("Hi!")
'centered: Hi! '
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Python Programming, 3/e 80
Better Change Counter n With what we know now about floating
point numbers, we might be uneasy about using them in a money situation.
n One way around this problem is to keep track of money in cents using an int or long int, and convert it into dollars and cents when output.
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Better Change Counter n If total is a value in cents (an int), dollars = total//100 cents = total%100
n Cents is printed using width 0>2 to right-justify it with leading 0s (if necessary) into a field of width 2.
n Thus 5 cents becomes '05'
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Better Change Counter # change2.py # A program to calculate the value of some change in dollars. # This version represents the total cash in cents. def main(): print ("Change Counter\n") print ("Please enter the count of each coin type.") quarters = int(input("Quarters: ")) dimes = int(input("Dimes: ")) nickels = int(input("Nickels: ")) pennies = int(input("Pennies: ")) total = quarters * 25 + dimes * 10 + nickels * 5 + pennies print ("The total value of your change is ${0}.{1:0>2}" .format(total//100, total%100))
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Better Change Counter >>> main() Change Counter
Please enter the count of each coin type.
Quarters: 0
Dimes: 0
Nickels: 0
Pennies: 1
The total value of your change is $0.01
>>> main() Change Counter
Please enter the count of each coin type.
Quarters: 12
Dimes: 1
Nickels: 0
Pennies: 4
The total value of your change is $3.14
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n A file is a sequence of data that is stored in secondary memory (disk drive).
n Files can contain any data type, but the easiest to work with are text.
n A file usually contains more than one line of text.
n Python uses the standard newline character (\n) to mark line breaks.
Files: Multi-line Strings
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Multi-Line Strings n Hello World Goodbye 32
n When stored in a file: Hello\nWorld\n\nGoodbye 32\n
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Multi-Line Strings n This is exactly the same thing as
embedding \n in print statements. n Remember, these special characters
only affect things when printed. They don’t do anything during evaluation.
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File Processing n The process of opening a file involves
associating a file on disk with an object in memory.
n We can manipulate the file by manipulating this object. n Read from the file n Write to the file
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File Processing n When done with the file, it needs to be
closed. Closing the file causes any outstanding operations and other bookkeeping for the file to be completed.
n In some cases, not properly closing a file could result in data loss.
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File Processing n Reading a file into a word processor
n File opened n Contents read into RAM n File closed n Changes to the file are made to the copy
stored in memory, not on the disk.
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File Processing n Saving a word processing file
n The original file on the disk is reopened in a mode that will allow writing (this actually erases the old contents)
n File writing operations copy the version of the document in memory to the disk
n The file is closed
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File Processing n Working with text files in Python
n Associate a disk file with a file object using the open function <filevar> = open(<name>, <mode>)
n name is a string with the actual file name on the disk. The mode is either ‘r’ or ‘w’ depending on whether we are reading or writing the file.
n infile = open("numbers.dat", "r")
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File Methods n <file>.read() – returns the entire remaining
contents of the file as a single (possibly large, multi-line) string
n <file>.readline() – returns the next line of the file. This is all text up to and including the next newline character
n <file>.readlines() – returns a list of the remaining lines in the file. Each list item is a single line including the newline characters.
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File Processing # printfile.py # Prints a file to the screen. def main(): fname = input("Enter filename: ") infile = open(fname,'r') data = infile.read() print(data)
n First, prompt the user for a file name n Open the file for reading n The file is read as one string and stored in the
variable data
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File Processing n readline can be used to read the next
line from a file, including the trailing newline character infile = open(someFile, "r") for i in range(5): line = infile.readline() print line[:-1]
n This reads the first 5 lines of a file n Slicing is used to strip out the newline
characters at the ends of the lines
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File Processing n Another way to loop through the
contents of a file is to read it in with readlines and then loop through the resulting list. infile = open(someFile, "r") for line in infile.readlines(): # Line processing here
infile.close()
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File Processing n Python treats the file itself as a
sequence of lines! infile = open(someFile, "r") for line in infile: # process the line here
infile.close()
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File Processing n Opening a file for writing prepares the file
to receive data n If you open an existing file for writing,
you wipe out the file’s contents. If the named file does not exist, a new one is created. outfile = open("mydata.out", "w") print(<expressions>, file=outfile)
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Example Program: Batch Usernames
n Batch mode processing is where program input and output are done through files (the program is not designed to be interactive)
n Let’s create usernames for a computer system where the first and last names come from an input file.
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Example Program: Batch Usernames
# userfile.py # Program to create a file of usernames in batch mode. def main(): print ("This program creates a file of usernames from a") print ("file of names.") # get the file names infileName = input("What file are the names in? ") outfileName = input("What file should the usernames go in? ") # open the files infile = open(infileName, 'r') outfile = open(outfileName, 'w')
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Example Program: Batch Usernames # process each line of the input file for line in infile: # get the first and last names from line first, last = line.split() # create a username uname = (first[0]+last[:7]).lower() # write it to the output file print(uname, file=outfile) # close both files infile.close() outfile.close() print("Usernames have been written to", outfileName)
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Example Program: Batch Usernames n Things to note:
n It’s not unusual for programs to have multiple files open for reading and writing at the same time.
n The lower method is used to convert the names into all lower case, in the event the names are mixed upper and lower case.
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File Dialogs n A common problem with file manipulation
programs is figuring out exactly how to specify the file that you want to use.
n With no additional information, Python will look in the “current” directory for files.
n Most modern operating systems use file names having a form like <name>.<type> where type is a short indicator of what the file contains, e.g. txt (text file).
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File Dialogs n One problem: some operating systems
(Windows and MacOS) by default only show the part of the name preceeding the period, so it can be hard to figure out the complete file name.
n It’s even harder when the file is located somewhere other than the current directory in your secondary memory! Then we will need the complete path in addition to the file name.
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File Dialogs n On Windows, the complete file name may
look like C:/users/susan/Documents/Python_Programs/users.txt
n The solution? Allow the users to browse the file system visually and navigate to the file.
n This is a common enough operation that most operating systems provide a standard way to do this, usually incorporating a dialog box.
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File Dialogs n To ask the user for the name of a file to
open, you can use askopenfilename from tkinter.filedialog.
from tkinter.filedialog import askopenfilename
… infileName = aksopenfilename()
infile = open(infileName, "r")
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File Dialogs
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File Dialogs n When the user clicks the “Open” button, the
complete path name of the file is returned as a string and saved into the variable infileName.
n If the user clicks “Cancel”, the function returns an empty string.
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File Dialogs n To ask the user for the name of a file to
save, you can use asksaveasfilename from tkinter.filedialog.
from tkinter.filedialog import asksaveasfilename
…
outfileName = asksaveasfilename()
outfile = open(outfileName, "w")
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File Dialogs