Python 3 Cheat Sheet

Update (Nov 19 2018): Added exceptions and classes.

I’ve created this Python 3 cheat sheet to help beginners remember Python language syntax. You can also download this cheat sheet as a beautiful PDF here.

NOTE: This cheat sheet is a work in progress and is not complete yet. I’ll be adding new stuff to it over the next few weeks. So, be sure to come back and get the latest version.

If you’re starting out with Python and are looking for a fun and comprehensive tutorial, check out my YouTube tutorials. I have two Python tutorials. If you have no or little programming experience, I suggest you check out my Python tutorial for beginners. Otherwise, if you know the basics (eg variables, functions, conditional statements, loops) and are looking for a tutorial that gets straight to the point and doesn’t treat you like a beginner, check out my Python tutorial for programmers.

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Variables

a = 1       # integer
b = 1.1     # float
c = 1 + 2j  # complex number (a + bi)
d = “a”     # string
e = True    # boolean (True / False)

Strings

x = “Python”
len(x)
x[0]
x[-1]
x[0:3]

# Formatted strings
name = f”{first} {last}”

# Escape sequences
\” \’ \\ \n

# String methods
x.upper()
x.lower()
x.title()
x.strip()
x.find(“p”)
x.replace(“a”, “b”)
“a” in x

Type Conversion

int(x)  
float(x) 
bool(x) 
string(x)

Falsy Values

0
“”
[]

Conditional Statements

if x == 1:  
    print(“a”)
elif x == 2:  
    print(“b”)
else:   
    print(“c”)

# Ternary operator 
x = “a” if n > 1 else “b”

# Chaining comparison operators
if 18 <= age < 65:

Loops

for n in range(1, 10): 
    print(n)

while n < 10: 
    print(n)
    n += 1

Functions

def increment(number, by=1):   
    return number + by

# Keyword arguments 
increment(2, by=1)

# Variable number of arguments 
def multiply(*numbers): 
    for number in numbers: 
        print number 


multiply(1, 2, 3, 4)

# Variable number of keyword arguments 
def save_user(**user):  
    ...


save_user(id=1, name="Mosh")

Lists

# Creating lists
letters = ["a", "b", "c"]     
matrix = [[0, 1], [1, 2]]
zeros = [0] * 5
combined = zeros + letters
numbers = list(range(20))

# Accessing items
letters = ["a", "b", "c", "d"]
letters[0]  # "a"
letters[-1] # "d"

# Slicing lists 
letters[0:3]   # "a", "b", "c"
letters[:3]    # "a", "b", "c"
letters[0:]    # "a", "b", "c", "d"
letters[:]     # "a", "b", "c", "d"
letters[::2]   # "a", "c"
letters[::-1]  # "d", "c", "b", "a" 

# Unpacking 
first, second, *other = letters 

# Looping over lists 
for letter in letters: 
    ... 

for index, letter in enumerate(letters): 
    ... 

# Adding items 
letters.append("e")
letters.insert(0, "-")

# Removing items 
letters.pop()
letters.pop(0)
letters.remove("b")
del letters[0:3]

# Finding items 
if "f" in letters: 
    letters.index("f")

# Sorting lists 
letters.sort()
letters.sort(reverse=True) 

# Custom sorting 
items = [
    ("Product1", 10),
    ("Product2", 9),
    ("Product3", 11)
]

items.sort(key=lambda item: item[1])

# Map and filter 
prices = list(map(lambda item: item[1], items))
expensive_items = list(filter(lambda item: item[1] >= 10, items))

# List comprehensions 
prices = [item[1] for item in items]
expensive_items = [item for item in items if item[1] >= 10]

# Zip function 
list1 = [1, 2, 3]
list2 = [10, 20, 30]
combined = list(zip(list1, list2))    # [(1, 10), (2, 20)]

Tuples

point = (1, 2, 3)
point(0:2)     # (1, 2)
x, y, z = point 
if 10 in point: 
    ... 

# Swapping variables 
x = 10
y = 11
x, y = y, x 

Arrays

from array import array 

numbers = array("i", [1, 2, 3])

Sets

first = {1, 2, 3, 4}
second = {1, 5}

first | second  # {1, 2, 3, 4, 5}
first & second  # {1}
first - second  # {2, 3, 4}
first ^ second  # {2, 3, 4, 5}

if 1 in first: 
    ... 

Dictionaries

point = {"x": 1, "y": 2}
point = dict(x=1, y=2)
point["z"] = 3
if "a" in point: 
    ... 
point.get("a", 0)   # 0
del point["x"]
for key, value in point.items(): 
   ... 

# Dictionary comprehensions 
values = {x: x * 2 for x in range(5)}

Generator Expressions

values = (x * 2 for x in range(10000))
len(values)  # Error
for x in values: 

Unpacking Operator

first = [1, 2, 3]
second = [4, 5, 6]
combined = [*first, "a", *second]

first = {"x": 1}
second = {"y": 2}
combined = {**first, **second}

Exceptions

# Handling Exceptions 
try: 
  …

except (ValueError, ZeroDivisionError):
  …
else: 
  # no exceptions raised
finally:
  # cleanup code 

# Raising exceptions 
if x < 1: 
    
    raise ValueError(“…”)

# The with statement 
with open(“file.txt”) as file: 
   
   … 

Classes

# Creating classes
class Point: 
    def __init__(self, x, y): 
        
        self.x = x
        self.y = y 

    def draw(self): 
        
        …

# Instance vs class attributes
class Point: 
    default_color = “red”

    def __init__(self, x, y): 
        
        self.x = x

# Instance vs class methods
class Point: 
    def draw(self): 
       
        …
    
    @classmethod 
    def zero(cls): 
        
        return cls(0, 0)


# Magic methods
__str__()

__eq__()
__cmp__()
... 

# Private members 
class Point: 
    def __init__(self, x): 
        
        self.__x = x


# Properties 
class Point: 
    def __init__(self, x): 
        
        self.__x = x

    @property
    def x(self):    
        return self.__x     

    @property.setter:
    def x.setter(self, value): 
        self.__x = value 


# Inheritance
class FileStream(Stream): 
    def open(self): 
       
         super().open()
         … 

# Multiple inheritance 
class FlyingFish(Flyer, Swimmer): 
    … 

# Abstract base classes
from abc import ABC, abstractmethod

class Stream(ABC): 
    @abstractmethod
    def read(self): 
        pass  

# Named tuples 
from collections import namedtuple

Point = namedtuple(“Point”, [“x”, “y”])
point = Point(x=1, y=2)

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