Context managers

A context manager sets something up and guarantees it gets cleaned up, even if an error happens halfway through. with open(...) as f: closes the file for you. Write your own with a class that has __enter__ and __exit__, or more simply with @contextmanager from contextlib around a generator: the code before yield is setup, and the code after it is cleanup.

How it reads

• Everything before yield runs when the with block starts
• The yielded value becomes the name after as
• finally makes sure the cleanup runs even if the block raises

Cloud tip: Reach for a context manager whenever something must be undone: closing files, releasing locks, restoring settings.