Python 3 Cheat Sheet | 2021

I’ve created this Python 3.9 cheat sheet to help beginners learn the syntax of the Python Programming language

Variables

a = 23      # integer
b = 23.3    # float
c = 2+ j  # complex number (a + bi)
d = "Hello"    # string
e = True   # boolean (True / False)

Strings

str = “Hello Python”
len(str)
str[0]
str[-1]
str[0:3]
# Formatted strings
name = f”{first} {last}”
# Escape sequences
\” \’ \\ \n
# String methods
str.upper()
str.lower()
str.title()
str.strip()
str.find(“p”)
str.replace(“a”, “b”)
“a” in str

Type Conversion

int(a)
float(a)
bool(a)
string(a)

Falsy Values

0
“”
[]

Conditional Statements

if num == 1:
print(“1”)
elif num == 2:
print(“1”)
else:
print(“0”)
# 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="Deep")

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)