feature_04_contextlib_magic.md

4. Contextlib Context Managers

Concept summary

contextlib.contextmanager lets you define with-block behavior using a function with yield. Code before yield runs on enter, and code in finally runs on exit.

What C# developers usually expect

In C#, this is closest to using / IDisposable lifecycle control. Python can do class-based context managers too, but @contextmanager provides a compact function-based style.

How this example works

banner("deploy") prints start:deploy, then yields control to the with block. Inside the block, inside is printed. When the block exits, the finally section runs and prints end:deploy. This happens even if an exception occurs inside the block.

Why this matters in production

Use this for setup/cleanup boundaries such as timing, tracing, temporary environment changes, lock management, or transactional wrappers. It keeps resource and lifecycle logic centralized.

Common mistakes

Forgetting try/finally around yield, which can skip cleanup on errors. Putting too much business logic inside the context manager instead of keeping it focused on lifecycle concerns.

Source

• feature_04_contextlib_magic.py

Exact example from code file

from contextlib import contextmanager


@contextmanager
def banner(label: str):
    print(f"start:{label}")
    try:
        yield
    finally:
        print(f"end:{label}")


def main() -> None:
    with banner("deploy"):
        print("inside")


if __name__ == "__main__":
    main()

Run

uv run src/csharp_to_python_learning/python_only_features/feature_04_contextlib_magic.py

Expected output

• start:deploy
• inside
• end:deploy