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Python Cheat Sheet

Basic cheatsheet for Python mostly based on the book written by Al Sweigart, Automate the Boring Stuff with Python under the Creative Commons license and many other sources.

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Functions

def hello(name):
    print('Hello {}'.format(name))

Return Values and return Statements

When creating a function using the def statement, you can specify what the return value should be with a return statement. A return statement consists of the following:

  • The return keyword.

  • The value or expression that the function should return.

import random
def getAnswer(answerNumber):
    if answerNumber == 1:
        return 'It is certain'
    elif answerNumber == 2:
        return 'It is decidedly so'
    elif answerNumber == 3:
        return 'Yes'
    elif answerNumber == 4:
        return 'Reply hazy try again'
    elif answerNumber == 5:
        return 'Ask again later'
    elif answerNumber == 6:
        return 'Concentrate and ask again'
    elif answerNumber == 7:
        return 'My reply is no'
    elif answerNumber == 8:
        return 'Outlook not so good'
    elif answerNumber == 9:
        return 'Very doubtful'

r = random.randint(1, 9)
fortune = getAnswer(r)
print(fortune)

The None Value

spam = print('Hello!')
spam is None

Note: never compare to None with the == operator. Always use is.

print Keyword Arguments

print('Hello', end='')
print('World')
print('cats', 'dogs', 'mice')
print('cats', 'dogs', 'mice', sep=',')

Local and Global Scope

  • Code in the global scope cannot use any local variables.

  • However, a local scope can access global variables.

  • Code in a function’s local scope cannot use variables in any other local scope.

  • You can use the same name for different variables if they are in different scopes. That is, there can be a local variable named spam and a global variable also named spam.

The global Statement

If you need to modify a global variable from within a function, use the global statement:

def spam():
    global eggs
    eggs = 'spam'

eggs = 'global'
spam()
print(eggs)

There are four rules to tell whether a variable is in a local scope or global scope:

  1. If a variable is being used in the global scope (that is, outside of all functions), then it is always a global variable.

  2. If there is a global statement for that variable in a function, it is a global variable.

  3. Otherwise, if the variable is used in an assignment statement in the function, it is a local variable.

  4. But if the variable is not used in an assignment statement, it is a global variable.
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