How do Python context managers work?

Direct answer

What a strong answer covers

How it works

The with statement calls __enter__ on entry, binds its return value to the target, and guarantees __exit__ runs on the way out, whether by normal completion, return, or exception. __exit__ receives the exception details and can suppress the error by returning True, which is rarely wanted. contextlib.contextmanager turns a generator with one yield into a context manager, and ExitStack composes a variable number of them. The pattern applies to anything with setup and teardown, not only files and locks.

Example

A database session wrapper opens a transaction in __enter__, commits in __exit__ when no exception occurred, and rolls back otherwise, so business code stays free of try and finally. A test helper written with contextmanager temporarily overrides an environment variable and restores it even when the assertion fails.

Common mistakes

• Returning True from __exit__ by accident and silently swallowing every exception.
• Acquiring a resource in __init__ instead of __enter__, so the resource exists before the with block.
• Nesting many with statements manually when ExitStack would keep the code flat.

A useful answer structure

Answer from your own work.

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