COMP 118: Procedural Programming. Fall 2003.

Python Cheat Sheet

When learning about a new technical area there's often a bewildering amount of detail to keep track of. Generations of techies have made use of cheat sheets to have an easy reference to these details.
A more detailed alternative is provided by the Python Quick Reference. If you want to print it out or save it locally several forms are available


Arithmetic · Strings · Assignment · print · Getting Input · if · while · Play again? · Lists

· Text file processing · Dictionaries


Constants (Number types):



Constants: "This is a string"



Like lists Python strings are a sequence type so many list commands also work with strings.

Testing string contents:

Finding things in strings:

Changing strings. Remember that strings are immutable so to make a change "stick" you have to do, e.g. s = s.title().

String to list:

List to string:


Use = to assign a name to a value, e.g. distance = 48.1, name = "Tim Topper". Remember that the name has to be on the left hand side of the =, i.e. 48.1 = distance is an error.


Use print to display stored values.

print list

Displays the value of each item in the list. Puts a space between each pair of values. Example: print "The answer is", 5 + 2, "." displays:

The answer is 7 .

Appending a comma to the end of a print statement holds the current output line open, e.g. the code

print "The answer is",
print 5 + 2, "."


The answer is 7 .

on a single line.

For more control over output appearance embed formatting codes into output strings. See section String Formatting Operations of the Python Library Reference for the gory details.

Getting Input

Use input to get numerical data from the user,

input( string )

and raw_input to get string data.

Both display string (if given) and then read a line of input, by default from the keyboard. The difference is that raw_input just returns the string, while input evaluates it as a Python exprssion and returns the result.


distance = input( "Enter the distance in miles: "
name = raw_input( "What is your name? " )

N.B. the spaces before the second " in each case.


Use if to execute one block of code or another, but not both.

if test:
elif test:

N.B. the elif and else statements are optional as shown in the first two examples below.


if x < 0: print x, "is negative"
if flip == 1:
    print "You got heads"
    print "You got tails"
if num < 0:
    print "The number", num, "is negative."
elif num == 0:
    print "The number", num, "is neither positive nor negative."
    print "The number", num, "is positive."


Use while to execute a block of code multiple times.

while test:


x = 1
while x < 10
    print x
    x = x + 1
num = input( "Enter a number between 1 and 100: " )
while num < 1 or num > 100:
    print "Oops, your input value (", num, ") is out of range."
    num = input( "Be sure to enter a value between 1 and 100: " )

Play again? Repeating a program

again = "y"
while again == "y" or again == "Y" or again == "yes" or again == "Yes":

    # Put the body of your program here

    again = raw_input( "Play again (y/n)? " )

print "Thanks for playing"


Unlike many languages Python provides a built-in list type. A list constant is just a list of items separated by commas and placed inside square brackets, e.g. [ "Tim", 42, "Molly" ].

Python provides for a wealth of list operations (complete list in reference manual):

N.B. the elements in lists are numbered from 0, not 1.

Text file processing

To read from a file a line at a time:

infilename = raw_input( "Name of file to read from: " )
infile = open( infilename, "r" )
for line in infile:
  # Do stuff with line.
  # Remember that line is a string even if it looks like a number,
  # e.g. num = int( line )

There's more than this of course. You can also read the entire file into a string in one fell swoop using, read the entire file into a list of strings (one per line in the file) using infile.readlines(), or read a certain number of bytes using N ) where N gives the number of bytes to read.

To write to a file:

outfilename = raw_input( "Name of file to write to: " )
outfile = open( outfilename, "w" )
print >> outfile, ...

For more see the Python library reference on File Objects.


Python provides a built-in lookup table type it calls a dictionary (often called hash tables in other languages).

A dictionary constant consists of a series of key-value pairs enclosed by curly braces, e.g. d = { 'Tim' : 775, 'Brian' : 869 }. This creates a dictionary we can visualize as:

Common dictionary operations include: