Python (programming language)/Cheat sheet

Contents

The cheat sheet of Python

Scope: Python 3.10+, scripting, packaging, typing, async, testing, and common stdlib patterns.

0. Quick Start

Verify Python:

python3 --version

Create virtual environment:

python3 -m venv .venv
source .venv/bin/activate

Install package:

pip install requests

1. Project Layout (Common)

project/
├─ src/
│  └─ app/
│     └─ __init__.py
├─ tests/
├─ pyproject.toml
└─ README.md

2. Python Basics

Variables:

x = 10
name = "alice"

Multiple assignment:

a, b = 1, 2

3. Control Flow

If:

if x > 0:
    pass
elif x == 0:
    pass

For:

for i in range(5):
    print(i)

While:

while condition:
    break

4. Functions

def add(a: int, b: int) -> int:
    return a + b

Defaults:

def greet(name="world"):
    print(name)

Keyword-only:

def f(*, debug=False):
    pass

5. Data Structures

List:

xs = [1, 2, 3]

Dict:

m = {"a": 1}

Set:

s = {1, 2, 3}

Tuple:

t = (1, 2)

6. Comprehensions

[x*x for x in range(5)]
{x: x*x for x in range(5)}

Filtered:

[x for x in xs if x > 2]

7. Classes

class User:
    def __init__(self, name: str):
        self.name = name

8. Dataclasses

from dataclasses import dataclass

@dataclass
class User:
    id: int
    name: str

9. Exceptions

try:
    x = int("a")
except ValueError:
    pass

Raise:

raise RuntimeError("oops")

10. File I/O

with open("file.txt") as f:
    data = f.read()

Write:

with open("file.txt", "w") as f:
    f.write("hi")

11. Modules & Imports

import math
from pathlib import Path

Relative import:

from .utils import helper

12. Packaging (`pyproject.toml`)

Minimal:

[project]
name = "myapp"
version = "0.1.0"
dependencies = ["requests"]

Install locally:

pip install -e .

13. Typing

def f(x: int) -> str:
    return str(x)

Collections:

list[int]
dict[str, int]

Optional:

int | None

14. Async Programming

Coroutine:

async def fetch():
    return 1

Run:

import asyncio
asyncio.run(fetch())

Gather:

await asyncio.gather(a(), b())

15. JSON

import json

json.dumps(obj)
json.loads(s)

16. CLI Arguments

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("--port", type=int)
args = parser.parse_args()

17. Logging

import logging
logging.basicConfig(level=logging.INFO)

18. Testing (pytest)