Python Cheat Sheet

$ python 

>>> exit() 

$ python my_script.py 

$ python -i my_script.py 

# This is a comment # print("This code will not run.") print("This will run.") # Comments are ignored by Python 

type(42) # <class 'int'> type(3.14) # <class 'float'> type("Hello") # <class 'str'> type(True) # <class 'bool'> type(None) # <class 'NoneType'> isinstance(3.14, float) # True issubclass(int, object) # True - everything inherits from object 

int("42") # 42 float("3.14") # 3.14 str(42) # "42" bool(1) # True list("abc") # ["a", "b", "c"] 

name = "Leo" # String age = 7 # Integer height = 5.6 # Float is_cat = True # Boolean flaws = None # None type 

x, y = 10, 20 # Assign multiple values a = b = c = 0 # Give same value to multiple variables 

counter += 1 numbers += [4, 5] permissions |= write 

single = 'Hello' double = "World" multi = """Multiple line string""" 

greeting = "me" + "ow!" # "meow!" repeat = "Meow!" * 3 # "Meow!Meow!Meow!" length = len("Python") # 6 

"a".upper() # "A" "A".lower() # "a" " a ".strip() # "a" "abc".replace("bc", "ha") # "aha" "a b".split() # ["a", "b"] "-".join(["a", "b"]) # "a-b" 

text = "Python" text[0] # "P" (first) text[-1] # "n" (last) text[1:4] # "yth" (slice) text[:3] # "Pyt" (from start) text[3:] # "hon" (to end) text[::2] # "Pto" (every 2nd) text[::-1] # "nohtyP" (reverse) 

# f-strings name = "Aubrey" age = 2 f"Hello, {name}!" # "Hello, Aubrey!" f"{name} is {age} years old" # "Aubrey is 2 years old" f"Debug: {age=}" # "Debug: age=2" # Format method template = "Hello, {name}! You're {age}." template.format(name="Aubrey", age=2) # "Hello, Aubrey! You're 2." 

# Normal string with an escaped tab "This is:\tCool." # "This is: Cool." # Raw string with escape sequences r"This is:\tCool." # "This is:\tCool." 

10 + 3 # 13 10 - 3 # 7 10 * 3 # 30 10 / 3 # 3.3333333333333335 10 // 3 # 3 10 % 3 # 1 2 ** 3 # 8 

abs(-5) # 5 round(3.7) # 4 round(3.14159, 2) # 3.14 min(3, 1, 2) # 1 max(3, 1, 2) # 3 sum([1, 2, 3]) # 6 

if age < 13: category = "child" elif age < 20: category = "teenager" else: category = "adult" 

x == y # Equal to x != y # Not equal to x < y # Less than x <= y # Less than or equal x > y # Greater than x >= y # Greater than or equal 

if age >= 18 and has_car: print("Roadtrip!") if is_weekend or is_holiday: print("No work today.") if not is_raining: print("You can go outside.") 

# Loop through range for i in range(5): # 0, 1, 2, 3, 4 print(i) # Loop through collection fruits = ["apple", "banana"] for fruit in fruits: print(fruit) # With enumerate for index for i, fruit in enumerate(fruits): print(f"{i}: {fruit}") 

while True: user_input = input("Enter 'quit' to exit: ") if user_input == "quit": break print(f"You entered: {user_input}") 

for i in range(10): if i == 3: continue # Skip this iteration if i == 7: break # Exit loop print(i) 

def greet(): return "Hello!" def greet_person(name): return f"Hello, {name}!" def add(x, y=10): # Default parameter return x + y 

greet() # "Hello!" greet_person("Bartosz") # "Hello, Bartosz" add(5, 3) # 8 add(7) # 17 

def get_min_max(numbers): return min(numbers), max(numbers) minimum, maximum = get_min_max([1, 5, 3]) 

callable() # Checks if an object can be called as a function dir() # Lists attributes and methods globals() # Get a dictionary of the current global symbol table hash() # Get the hash value id() # Get the unique identifier locals() # Get a dictionary of the current local symbol table repr() # Get a string representation for debugging 

square = lambda x: x**2 result = square(5) # 25 # With map and filter numbers = [1, 2, 3, 4] squared = list(map(lambda x: x**2, numbers)) evens = list(filter(lambda x: x % 2 == 0, numbers)) 

class Dog: def __init__(self, name, age): self.name = name self.age = age def bark(self): return f"{self.name} says Woof!" # Create instance my_dog = Dog("Frieda", 3) print(my_dog.bark()) # Frieda says Woof! 

class Cat: species = "Felis catus" # Class attribute def __init__(self, name): self.name = name # Instance attribute def meow(self): return f"{self.name} says Meow!" @classmethod def create_kitten(cls, name): return cls(f"Baby {name}") 

class Animal: def __init__(self, name): self.name = name def speak(self): pass class Dog(Animal): def speak(self): return f"{self.name} barks!" 

try: number = int(input("Enter a number: ")) result = 10 / number except ValueError: print("That's not a valid number!") except ZeroDivisionError: print("Cannot divide by zero!") else: print(f"Result: {result}") finally: print("Calculation attempted") 

ValueError # Invalid value TypeError # Wrong type IndexError # List index out of range KeyError # Dict key not found FileNotFoundError # File doesn't exist 

def validate_age(age): if age < 0: raise ValueError("Age cannot be negative") return age 

# Creating lists empty = [] nums = [5] mixed = [1, "two", 3.0, True] # List methods nums.append("x") # Add to end nums.insert(0, "y") # Insert at index 0 nums.extend(["z", 5]) # Extend with iterable nums.remove("x") # Remove first "x" last = nums.pop() # Pop returns last element # List indexing and checks fruits = ["banana", "apple", "orange"] fruits[0] # "banana" fruits[-1] # "orange" "apple" in fruits # True len(fruits) # 3 

# Creating tuples point = (3, 4) single = (1,) # Note the comma! empty = () # Basic tuple unpacking point = (3, 4) x, y = point x # 3 y # 4 # Extended unpacking first, *rest = (1, 2, 3, 4) first # 1 rest # [2, 3, 4] 

# Creating Sets a = {1, 2, 3} b = set([3, 4, 4, 5]) # Set Operations a | b # {1, 2, 3, 4, 5} a & b # {3} a - b # {1, 2} a ^ b # {1, 2, 4, 5} 

# Creating Dictionaries empty = {} pet = {"name": "Leo", "age": 42} # Dictionary Operations pet["sound"] = "Purr!" # Add key and value pet["age"] = 7 # Update value age = pet.get("age", 0) # Get with default del pet["sound"] # Delete key pet.pop("age") # Remove and return # Dictionary Methods pet = {"name": "Frieda", "sound": "Bark!"} pet.keys() # dict_keys(['name', 'sound']) pet.values() # dict_values(['Frieda', 'Bark!']) pet.items() # dict_items([('name', 'Frieda'), ('sound', 'Bark!')]) 

# Basic squares = [x**2 for x in range(10)] # With condition evens = [x for x in range(20) if x % 2 == 0] # Nested matrix = [[i*j for j in range(3)] for i in range(3)] 

# Dictionary comprehension word_lengths = {word: len(word) for word in ["hello", "world"]} # Set comprehension unique_lengths = {len(word) for word in ["who", "what", "why"]} # Generator expression sum_squares = sum(x**2 for x in range(1000)) 

# Read an entire file with open("file.txt", mode="r", encoding="utf-8") as file: content = file.read() # Read a file line by line with open("file.txt", mode="r", encoding="utf-8") as file: for line in file: print(line.strip()) # Write a file with open("output.txt", mode="w", encoding="utf-8") as file: file.write("Hello, World!\n") # Append to a File with open("log.txt", mode="a", encoding="utf-8") as file: file.write("New log entry\n") 

# Import entire module import math result = math.sqrt(16) # Import specific function from math import sqrt result = sqrt(16) # Import with alias import numpy as np array = np.array([1, 2, 3]) # Import all (not recommended) from math import * 

# Import from package import package.module from package import module from package.subpackage import module # Import specific items from package.module import function, Class from package.module import name as alias 

$ python -m venv .venv 

PS> .venv\Scripts\activate 

$ source .venv/bin/activate 

(.venv) $ deactivate 

$ python -m pip install requests 

$ python -m pip freeze > requirements.txt $ python -m pip install -r requirements.txt 

# Swap variables a, b = b, a # Flatten a list of lists matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] flat = [item for sublist in matrix for item in sublist] # Remove duplicates unique_unordered = list(set(my_list)) # Remove duplicates, preserve order unique = list(dict.fromkeys(my_list)) # Count occurrences from collections import Counter counts = Counter(my_list)