python-cheatsheet

Basic Cheat Sheet for Python (PDF, Markdown and Jupyter Notebook)

Project maintained by Barry-Williams Hosted on GitHub Pages — Theme by mattgraham

About

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. I have modified this to remove what you do not need to know.

Read It

Python Cheatsheet

Python Basics

Symbols and terminology

symbol Name symbol Name
‘ Single Quote \ Backslash
“ Double Quote > Greater
! Exclamation < Less
# Hash : Colon
() Parentheses ; Semi Colon
[] Square bracket {} curly braket
* Asterisk . Dot
/ Slash , Comma

Parentheses () are used to assign code including strings, numbers, variables etc to a particular function

print("this string belongs to the print function")
int(input("this string belongs to the input function, the input function belongs to the int function")) 
# notice the 2 closing parenthesis                                                                   ^

quotation marks “” or ‘’ are used to enclose strings either double or single quotation marks can be used so long as the same are used on either side

print('I can use single or double quotes')
print("I'm going to use double quotes here because the single is used as an apostrophe')

comma , is used to seperate arguments to a function

range(5)
range (3, 8)
range (3, 8, 2)

The range function can take one, two or, three arguements white space For the most part spaces in your code do not matter you will notice above in the range command the first command had no space between range the parenthesis but the rest do There are only a few occasions it really matters if you include a space in your code or not and I will cover when that is the case

Math Operators

From Highest to Lowest precedence:

Operators Operation Example
/ Division 22 / 8 = 2.75
* Multiplication 3 * 3 = 9
- Subtraction 5 - 2 = 3
+ Addition 2 + 2 = 4

Examples of expressions in the interactive shell:

>>> 2 + 3 * 6
20

>>> (2 + 3) * 6
30

>>> (5 - 1) * ((7 + 1) / (3 - 1))
16.0

Return to the Top

Data Types

Data Type Examples
Integers -2, -1, 0, 1, 2, 3, 4, 5
Floating-point numbers -1.25, -1.0, --0.5, 0.0, 0.5, 1.0, 1.25
Strings 'a', 'aa', 'aaa', 'Hello!', '11 cats'

Return to the Top

String Concatenation and Replication

String concatenation:

>>> 'Alice' + 'Bob'
'AliceBob'

Note: Avoid + operator for string concatenation. Prefer fStrings except in some instances.

String Replication:

>>> 'Alice' * 5
'AliceAliceAliceAliceAlice'

Return to the Top

Variables

You can name a variable anything as long as it obeys the following rules:

  1. It can be only one word.
  2. It can use only letters, numbers, and the underscore (_) character.
  3. It can’t begin with a number.
  4. Variable name starting with an underscore (_) are considered as “unuseful`.

Example:

>>> spam = 'Hello'
>>> spam
'Hello'

>>> _spam = 'Hello'

_spam should not be used again in the code.

camelCase

Is used to join multiple words together without spaces or _ and make it readable Example:

>>> firstName = 'John'
>>> lastName = 'Smith'

Return to the Top

Comments

Inline comment:

# This is a comment

Multiline comment:

# This is a
# multiline comment

Code with a comment:

a = 1  # initialization

Please note the two spaces in front of the comment.

Docstring: Using 3 quotation marks you can define a multiple line comment or Docstring Start and end the docstring with 3 quotation marks of the same type


"""
This is a function docstring
With multiple
lines
"""
''' you can also
    put your quotation
    marks at the start and end of a line '''

Return to the Top

The print() Function

>>> print('Hello world!')
Hello world!

>>> a = 1
>>> print(f'Hello world! {a}')
Hello world! 1

Return to the Top

The input() Function

Example Code:

>>> print('What is your name?')   # ask for their name
>>> myName = input()
>>> print(f'It is good to meet you, {myName})
What is your name?
Al
It is good to meet you, Al

Return to the Top

The len() Function

Evaluates to the integer value of the number of characters in a string:

>>> len('hello')
5

Note: test of emptiness of strings, lists, dictionary, etc, should not use len, but prefer direct boolean evaluation.

>>> a = [1, 2, 3]
>>> if a:
>>>     print("the list is not empty!")

Return to the Top

The str(), int(), and float() Functions

Integer to String or Float:

>>> str(29)
'29'

>>> print('I am '+ str(29) +'  years old.')
I am 29 years old.

You can also use fstrings to do this without the str command

>>> str(-3.14)
'-3.14'

Float to Integer:

>>> int(7.7)
7

>>> int(7.7) + 1
8

Return to the Top

Flow Control

Comparison Operators

Operator Meaning
== Equal to
!= Not equal to
< Less than
> Greater Than
<= Less than or Equal to
>= Greater than or Equal to

These operators evaluate to True or False depending on the values you give them.

Examples:

>>> 42 == 42
True

>>> 40 == 42
False

>>> 'hello' == 'hello'
True

>>> 'hello' == 'Hello'
False

>>> 'dog' != 'cat'
True

>>> 42 == 42.0
True

>>> 42 == '42'
False

Boolean Operators

There are three Boolean operators: and, or, and not.

The and Operator’s Truth Table:

Expression Evaluates to
True and True True
True and False False
False and True False
False and False False

The or Operator’s Truth Table:

Expression Evaluates to
True or True True
True or False True
False or True True
False or False False

The not Operator’s Truth Table:

Expression Evaluates to
not True False
not False True

Return to the Top

Mixing Boolean and Comparison Operators

>>> (4 < 5) and (5 < 6)
True

>>> (4 < 5) and (9 < 6)
False

>>> (1 == 2) or (2 == 2)
True

You can also use multiple Boolean operators in an expression, along with the comparison operators:

>>> 2 + 2 == 4 and not 2 + 2 == 5 and 2 * 2 == 2 + 2
True

Return to the Top

Indentation and Code Blocks

When using the python keywords if, else elif, for, while, def, and you need to end the line with a : You then need to indent the lines of code you want to belong to that statement using the tab key or multiple spaces (tab is prefered)

if this == that:
  this line belongs to the if statement (line 1)
  so does this line (line 2)
this line does not (line 3)

What this means as we will see in the next section if this equals that line 1 and line 2 will run Line 3 will run regardless if this equals that

Any time you see a : followed by indented lines it means anything that is indented belongs to the statement with the :

if Statements

if name == 'Alice':
    print('Hi, Alice.')

Return to the Top

else Statements

name = 'Bob'
if name == 'Alice':
    print('Hi, Alice.')
else:
    print('Hello, stranger.')

Return to the Top

elif Statements

name = 'Bob'
age = 5
if name == 'Alice':
    print('Hi, Alice.')
elif age < 12:
    print('You are not Alice, kiddo.')

name = 'Bob'
age = 30
if name == 'Alice':
    print('Hi, Alice.')
elif age < 12:
    print('You are not Alice, kiddo.')
else:
    print('You are neither Alice nor a little kid.')

Return to the Top

while Loop Statements

spam = 0
while spam < 5:
    print('Hello, world.')
    spam = spam + 1

Return to the Top

break Statements

If the execution reaches a break statement, it immediately exits the while loop’s clause:

while True:
    print('Please type your name.')
    name = input()
    if name == 'your name':
        break
print('Thank you!')

Return to the Top

continue Statements

When the program execution reaches a continue statement, the program execution immediately jumps back to the start of the loop.

while True:
    print('Who are you?')
    name = input()
    if name != 'Joe':
        continue
    print('Hello, Joe. What is the password? (It is a fish.)')
    password = input()
    if password == 'swordfish':
        break
print('Access granted.')

Return to the Top

for Loops and the range() Function

>>> print('My name is')
>>> for i in range(5):
>>>     print(f'Jimmy Five Times ({i})')
My name is
Jimmy Five Times (0)
Jimmy Five Times (1)
Jimmy Five Times (2)
Jimmy Five Times (3)
Jimmy Five Times (4)

The range() function can also be called with three arguments. The first two arguments will be the start and stop values, and the third will be the step argument. The step is the amount that the variable is increased by after each iteration.

>>> for i in range(0, 10, 2):
>>>    print(i)
0
2
4
6
8

You can even use a negative number for the step argument to make the for loop count down instead of up.

>>> for i in range(5, -1, -1):
>>>     print(i)
5
4
3
2
1
0

For else statement

This allows to specify a statement to execute in case of the full loop has been executed. Only useful when a break condition can occur in the loop:

>>> for i in [1, 2, 3, 4, 5]:
>>>    if i == 3:
>>>        break
>>> else:
>>>    print("only executed when no item of the list is equal to 3")

Return to the Top

Importing Modules

import random
for i in range(5):
    print(random.randint(1, 10))

import random, sys, os, math

from random import *

Return to the Top

Ending a Program Early with sys.exit()

import sys

while True:
    print('Type exit to exit.')
    response = input()
    if response == 'exit':
        sys.exit()
    print(f'You typed {response}.')

Return to the Top

Functions

>>> def hello(name):
>>>     print(f'Hello {name}')
>>>
>>> hello('Alice')
>>> hello('Bob')
Hello Alice
Hello Bob

Return to the Top

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:

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)

Return to the Top

Return to the Top

Local and Global Scope

Return to the Top

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)
spam

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.

Return to the Top

Exception Handling

Basic exception handling

>>> def spam(divideBy):
>>>     try:
>>>         return 42 / divideBy
>>>     except ZeroDivisionError as e:
>>>         print(f'Error: Invalid argument: {e}')
>>>
>>> print(spam(2))
>>> print(spam(12))
>>> print(spam(0))
>>> print(spam(1))
21.0
3.5
Error: Invalid argument: division by zero
None
42.0

Return to the Top

Final code in exception handling

Code inside the finally section is always executed, no matter if an exception has been raised or not, and even if an exception is not caught.

>>> def spam(divideBy):
>>>     try:
>>>         return 42 / divideBy
>>>     except ZeroDivisionError as e:
>>>         print(f'Error: Invalid argument: {e}')
>>>     finally:
>>>         print("-- division finished --")
>>> print(spam(2))
-- division finished --
21.0
>>> print(spam(12))
-- division finished --
3.5
>>> print(spam(0))
Error: Invalid Argument division by zero
-- division finished --
None
>>> print(spam(1))
-- division finished --
42.0

Return to the Top

Lists

>>> spam = ['cat', 'bat', 'rat', 'elephant']

>>> spam
['cat', 'bat', 'rat', 'elephant']

Return to the Top

Getting Individual Values in a List with Indexes

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[0]
'cat'

>>> spam[1]
'bat'

>>> spam[2]
'rat'

>>> spam[3]
'elephant'

Return to the Top

Negative Indexes

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[-1]
'elephant'

>>> spam[-3]
'bat'

>>> f'The {spam[-1]} is afraid of the {spam[-3]}.')
'The elephant is afraid of the bat.'

Return to the Top

Getting Sublists with Slices

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[0:4]
['cat', 'bat', 'rat', 'elephant']

>>> spam[1:3]
['bat', 'rat']

>>> spam[0:-1]
['cat', 'bat', 'rat']

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[:2]
['cat', 'bat']

>>> spam[1:]
['bat', 'rat', 'elephant']

Slicing the complete list will perform a copy:

>>> spam2 = spam[:]
['cat', 'bat', 'rat', 'elephant']
>>> spam.append('dog')
>>> spam
['cat', 'bat', 'rat', 'elephant', 'dog']
>>> spam2
['cat', 'bat', 'rat', 'elephant']

Return to the Top

Getting a List’s Length with len()

>>> spam = ['cat', 'dog', 'moose']
>>> len(spam)
3

Return to the Top

Changing Values in a List with Indexes

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[1] = 'aardvark'

>>> spam
['cat', 'aardvark', 'rat', 'elephant']

>>> spam[2] = spam[1]

>>> spam
['cat', 'aardvark', 'aardvark', 'elephant']

>>> spam[-1] = 12345

>>> spam
['cat', 'aardvark', 'aardvark', 12345]

Return to the Top

Using for Loops with Lists

>>> supplies = ['pens', 'staplers', 'flame-throwers', 'binders']
>>> for i, supply in enumerate(supplies):
>>>     print(f'Index {i} in supplies is: {supply}')
Index 0 in supplies is: pens
Index 1 in supplies is: staplers
Index 2 in supplies is: flame-throwers
Index 3 in supplies is: binders

Return to the Top

The in and not in Operators

>>> 'howdy' in ['hello', 'hi', 'howdy', 'heyas']
True

>>> spam = ['hello', 'hi', 'howdy', 'heyas']
>>> 'cat' in spam
False

>>> 'howdy' not in spam
False

>>> 'cat' not in spam
True

Return to the Top

Augmented Assignment Operators

Operator Equivalent
spam += 1 spam = spam + 1
spam -= 1 spam = spam - 1
spam *= 1 spam = spam * 1
spam /= 1 spam = spam / 1
spam %= 1 spam = spam % 1

Examples:

>>> spam = 'Hello'
>>> spam += ' world!'
>>> spam
'Hello world!'

>>> bacon = ['Zophie']
>>> bacon *= 3
>>> bacon
['Zophie', 'Zophie', 'Zophie']

Return to the Top

Finding a Value in a List with the index() Method

>>> spam = ['Zophie', 'Pooka', 'Fat-tail', 'Pooka']

>>> spam.index('Pooka')
1

Return to the Top

Adding Values to Lists with the append() and insert() Methods

append():

>>> spam = ['cat', 'dog', 'bat']

>>> spam.append('moose')

>>> spam
['cat', 'dog', 'bat', 'moose']

insert():

>>> spam = ['cat', 'dog', 'bat']

>>> spam.insert(1, 'chicken')

>>> spam
['cat', 'chicken', 'dog', 'bat']

Return to the Top

Removing Values from Lists with remove()

>>> spam = ['cat', 'bat', 'rat', 'elephant']

>>> spam.remove('bat')

>>> spam
['cat', 'rat', 'elephant']

If the value appears multiple times in the list, only the first instance of the value will be removed.

Return to the Top

Removing Values from Lists with pop()

>>> spam = ['cat', 'bat', 'rat', 'elephant']

>>> spam.pop()
'elephant'

>>> spam
['cat', 'bat', 'rat']

>>> spam.pop(0)
'cat'

>>> spam
['bat', 'rat']

Return to the Top

Sorting the Values in a List with the sort() Method

>>> spam = [2, 5, 3.14, 1, -7]
>>> spam.sort()
>>> spam
[-7, 1, 2, 3.14, 5]

>>> spam = ['ants', 'cats', 'dogs', 'badgers', 'elephants']
>>> spam.sort()
>>> spam
['ants', 'badgers', 'cats', 'dogs', 'elephants']

You can also pass True for the reverse keyword argument to have sort() sort the values in reverse order:

>>> spam.sort(reverse=True)
>>> spam
['elephants', 'dogs', 'cats', 'badgers', 'ants']

If you need to sort the values in regular alphabetical order, pass str. lower for the key keyword argument in the sort() method call:

>>> spam = ['a', 'z', 'A', 'Z']
>>> spam.sort(key=str.lower)
>>> spam
['a', 'A', 'z', 'Z']

You can use the built-in function sorted to return a new list:

>>> spam = ['ants', 'cats', 'dogs', 'badgers', 'elephants']
>>> sorted(spam)
['ants', 'badgers', 'cats', 'dogs', 'elephants']

Return to the Top

Multiline Strings with Triple Quotes

>>> print('''Dear Alice,
>>>
>>> Eve's cat has been arrested for catnapping, cat burglary, and extortion.
>>>
>>> Sincerely,
>>> Bob''')
Dear Alice,

Eve's cat has been arrested for catnapping, cat burglary, and extortion.

Sincerely,
Bob

Indexing and Slicing Strings

H   e   l   l   o       w   o   r   l   d    !
0   1   2   3   4   5   6   7   8   9   10   11

>>> spam = 'Hello world!'

>>> spam[0]
'H'

>>> spam[4]
'o'

>>> spam[-1]
'!'

Slicing:


>>> spam[0:5]
'Hello'

>>> spam[:5]
'Hello'

>>> spam[6:]
'world!'

>>> spam[6:-1]
'world'

>>> spam[:-1]
'Hello world'

>>> spam[::-1]
'!dlrow olleH'

>>> spam = 'Hello world!'
>>> fizz = spam[0:5]
>>> fizz
'Hello'

Return to the Top

The in and not in Operators with list

>>> a = [1, 2, 3, 4]
>>> 5 in a
False

>>> 2 in a
True

Return to the Top

The upper(), lower(), isupper(), and islower() String Methods

upper() and lower():

>>> spam = 'Hello world!'
>>> spam = spam.upper()
>>> spam
'HELLO WORLD!'

>>> spam = spam.lower()
>>> spam
'hello world!'

>>> firstName = "john"
>>> firstName.capatilize()
'John'

isupper() and islower():

>>> spam = 'Hello world!'
>>> spam.islower()
False

>>> spam.isupper()
False

>>> 'HELLO'.isupper()
True

>>> 'abc12345'.islower()
True

>>> '12345'.islower()
False

>>> '12345'.isupper()
False

Return to the Top

Formatted String Literals or f-strings (Python 3.6+)

>>> name = 'Elizabeth'
>>> print (f'Hello {name}!')
'Hello Elizabeth!

It is even possible to do inline arithmetic with it:

>>> a = 5
>>> b = 10
>>> f'Five plus ten is {a + b} and not {2 * (a + b)}.'
print ('Five plus ten is 15 and not 30.')

fstings are one place where a space does matter you cannot put a space between the f and the quotation marks

Return to the Top