OS Process & Env

Python · Reference cheat sheet

📋 Overview

Deeper os usage: process ids, walk trees, permissions bits, and portable env handling for CLIs and services.

🔧 Core concepts

Theory: env is inherited by child processes. Mutating os.environ affects the current process only unless you pass env= to subprocess.

💡 Examples

Walk and filter:

import os

for root, dirs, files in os.walk("src"):
    dirs[:] = [d for d in dirs if d != "__pycache__"]
    for name in files:
        if name.endswith(".py"):
            print(os.path.join(root, name))

Stat file:

import os
import time

st = os.stat("README.md")
print(st.st_size, time.ctime(st.st_mtime))

Required env:

import os
import sys

def require(name: str) -> str:
    val = os.getenv(name)
    if not val:
        sys.exit(f"missing env {name}")
    return val

DATABASE_URL = require("DATABASE_URL")

⚠️ Pitfalls

• os.access has TOCTOU races — prefer try/except on open.
• walk mutates dirs in place to prune — a powerful but easy-to-miss trick.
• Never log full env dumps in production (secrets).

🔗 Related

• OS
• Subprocess
• Pathlib
• Dotenv