Which Python version should I learn?

Use a current supported Python 3 release. This sheet works on 3.10 or newer, and match statements need 3.10.

Python Cheat Sheet: Syntax, Collections, Functions, and Files

Introduction

Use this Python 3 cheat sheet when you need the syntax fast. Each section is a compact reference you can copy and adapt. For explanations and exercises, follow the Python lessons on this site.

Prerequisites

Python 3.10 or newer installed

Step-by-Step Guide

Step 1: Variables, types, and operators

Python is dynamically typed. Use type() to inspect a value and convert with int(), float(), str(), and bool().

name = "Ada"            # str
age = 36                # int
price = 9.99            # float
ok = True               # bool
nothing = None          # NoneType

x, y = 3, 4             # multiple assignment
x, y = y, x             # swap
10 // 3   # 3  floor division
10 % 3    # 1  remainder
2 ** 10   # 1024 power
a == b    # equal      a is b   # same object
and  or  not            # logical operators

Step 2: Strings

Strings are immutable. Use f-strings for formatting, and slicing for substrings.

s = "Hello, World"
f"{name} is {age} years old"     # f-string
f"{price:.2f}"                    # 2 decimals -> 9.99
s.lower(); s.upper(); s.strip()
s.split(", ")                     # ['Hello', 'World']
"-".join(["a", "b", "c"])         # 'a-b-c'
s.replace("World", "Python")
s.startswith("He"); "World" in s
s[0]; s[-1]; s[0:5]; s[::-1]      # index, last, slice, reverse
len(s)

Step 3: Lists, tuples, sets, and dictionaries

Lists are ordered and mutable, tuples are ordered and immutable, sets hold unique values, and dicts map keys to values.

nums = [3, 1, 2]
nums.append(4); nums.extend([5, 6]); nums.insert(0, 9)
nums.remove(9); nums.pop(); nums.sort(); nums.reverse()
sorted(nums, reverse=True)       # new list
point = (1, 2)                   # tuple
tags = {"a", "b"}                # set
tags.add("c"); tags & {"a"}      # union |, intersection &, difference -

user = {"name": "Ada", "age": 36}
user["email"] = "ada@example.com"
user.get("phone", "n/a")         # safe lookup with default
for key, value in user.items():
    print(key, value)
user.pop("age"); "name" in user

Step 4: Control flow and comprehensions

Indentation defines blocks. Comprehensions build lists, sets, and dicts in one line.

if age >= 18:
    print("adult")
elif age >= 13:
    print("teen")
else:
    print("child")

for i in range(3):               # 0, 1, 2
    print(i)
for i, item in enumerate(nums):
    print(i, item)
while age < 40:
    age += 1

squares = [n * n for n in range(10) if n % 2 == 0]
lookup = {n: n * n for n in range(5)}
unique = {n % 3 for n in range(10)}
match command:                    # Python 3.10+
    case "start": ...
    case "stop": ...
    case _: ...

Step 5: Functions and classes

Use type hints for readability. Dataclasses remove boilerplate for classes that mostly hold data.

def greet(name: str, punctuation: str = "!") -> str:
    return f"Hello, {name}{punctuation}"

def total(*numbers, **options):    # variable arguments
    return sum(numbers)

square = lambda n: n * n

from dataclasses import dataclass

@dataclass
class User:
    name: str
    age: int = 0

    def is_adult(self) -> bool:
        return self.age >= 18

u = User("Ada", 36)
print(u.is_adult())

Step 6: Files, JSON, and errors

Use with so files close automatically, pathlib for paths, and try/except for errors.

from pathlib import Path
import json

text = Path("notes.txt").read_text(encoding="utf-8")
Path("out.txt").write_text("hello", encoding="utf-8")

with open("data.json", encoding="utf-8") as f:
    data = json.load(f)
with open("out.json", "w", encoding="utf-8") as f:
    json.dump(data, f, indent=2)

try:
    value = int("abc")
except ValueError as error:
    print("bad number:", error)
else:
    print("ok")
finally:
    print("always runs")

raise ValueError("something went wrong")

Step 7: Environments and packages

Create a virtual environment for every project so dependencies stay separate.

python -m venv .venv
source .venv/bin/activate        # macOS/Linux
.venv\Scripts\activate           # Windows
pip install requests
pip freeze > requirements.txt
pip install -r requirements.txt
python -m pytest                  # run tests
python script.py

FAQs

Which Python version should I learn?: Use a current supported Python 3 release. This sheet works on 3.10 or newer, and match statements need 3.10.

Summary

Keep this sheet open while you code: f-strings, slicing, list and dict methods, comprehensions, dataclasses, pathlib, try/except, and virtual environments cover most everyday Python.