OS

Python · Reference cheat sheet

📋 Overview

The os module talks to the operating system: env vars, process info, and basic path helpers. Prefer pathlib for path work; keep os for env, cwd, and platform details.

🔧 Core concepts

Theory: The process environment is a string→string map inherited by children. Path APIs either return strings (os.path) or objects (pathlib). File-system calls can raise OSError subclasses (FileNotFoundError, PermissionError) — catch the specific type you can recover from.

os.path vs pathlib: os.path returns strings; Path objects support /, .read_text(), .glob(), and clearer APIs. Use pathlib for new code; os.path still appears in older APIs.

💡 Examples

Environment variables:

import os

home = os.getenv("HOME") or os.getenv("USERPROFILE")
os.environ["APP_ENV"] = "dev"
print(os.environ.get("APP_ENV", "prod"))
# del os.environ["APP_ENV"]  # unset

Cwd and listing:

import os
from pathlib import Path

print(os.getcwd())
for name in os.listdir("."):
    print(name, Path(name).is_file())

os.path vs pathlib:

import os
from pathlib import Path

s = os.path.join("data", "a.csv")
print(os.path.exists(s), os.path.splitext(s))

p = Path("data") / "a.csv"
print(p.exists(), p.suffix, p.read_text(encoding="utf-8") if p.exists() else "")

Create dirs (os vs pathlib):

import os
from pathlib import Path

os.makedirs("out/logs", exist_ok=True)
Path("out/cache").mkdir(parents=True, exist_ok=True)

Expand user + join portably:

import os
from pathlib import Path

cfg = Path(os.path.expanduser("~")) / ".config" / "myapp" / "config.toml"
print(cfg)

Replace extension safely:

import os

base, _ = os.path.splitext("report.CSV")
print(base + ".parquet")

Walk with prune:

import os

for root, dirs, files in os.walk("."):
    dirs[:] = [d for d in dirs if not d.startswith(".")]
    print(root, len(files))

⚠️ Pitfalls

• os.environ["MISSING"] raises KeyError — prefer getenv / .get.
• Env values are always strings; cast (int(os.getenv("PORT", "8000"))).
• chdir is process-global and surprises libraries — avoid in libraries.
• Mixing str and Path is fine at the boundary; stick to one style inside a module.
• listdir does not recurse; use Path.rglob or os.walk for trees.

🔗 Related

• Pathlib
• Files I/O
• Dotenv
• Shutil
• Sys
• OS process & env
• Venv