Python Cheat Sheet

A condensed overview of the Python programming language: syntax, data types, variables, strings, control flow, functions, classes, errors, I/O, and more.

Based on the Python Cheat Sheet from Real Python.

Getting Started

• Always add a space after the #
• Use comments to explain the "why" of your code

Data Types

• Python is dynamically typed
• Use None to represent missing or optional values
• Use type() to check an object's type
• Check for a specific type with isinstance()
• issubclass() checks if a class is a subclass

Variables & Assignment

• Variables are created when first assigned
• Use descriptive variable names
• Follow the snake_case convention

Strings

• Use double quotes for strings
• Use "\n" to create a line break in a string
• To write a backslash in a normal string, write "\\"

Numbers & Math

• Use +, -, *, and / for basic arithmetic
• // is floor division, % is modulo, and ** is exponentiation
• / always returns a float

Conditionals

• Python uses indentation for code blocks
• Use 4 spaces per indentation level

Loops

• range(5) generates 0 through 4
• Use enumerate() to get index and value
• break exits the loop, continue skips to the next iteration
• Be careful with while to avoid creating an infinite loop

Functions

• Define functions with def
• Always use () to call a function
• Add return to send values back
• Create anonymous functions with the lambda keyword

Classes

• Classes are blueprints for objects
• You can create multiple instances of one class
• Classes commonly encapsulate data
• Inside a class, you provide methods for interacting with the data
• .__init__() is the constructor method
• self refers to the instance

Exceptions

• When Python encounters an error, it creates an exception
• Use specific exception types when possible
• else runs if no exception occurred
• finally always runs, even after errors

Collections

• A collection is any container data structure that stores multiple items
• If an object is a collection, you can loop through it
• Strings are collections, too
• Use len() to get the size of a collection
• Check if an item is in a collection with the in keyword
• Collections may look similar, but each data structure solves specific needs

Comprehensions

• Think of comprehensions as condensed for loops
• Comprehensions are generally faster than equivalent loops

File I/O

• Open files with open(), ideally inside a with statement so they close automatically
• Use "r" to read, "w" to write, and "a" to append

Imports & Modules

• Prefer explicit imports over import *
• Use aliases for long module names
• Group imports: standard library, third-party libraries, user-defined modules

Virtual Environments

• Virtual environments are often called "venv"
• Use venvs to isolate project packages from the system-wide Python packages

Packages

• The official third-party package repository is the Python Package Index (PyPI)

Miscellaneous

Truthy	Falsy
-42	0
3.14	0.0
"John"	""
[1, 2, 3]	[]
("apple", "banana")	()
{"key": None}	{}
	None