How do context managers work, and when have you written your own?

A context manager is any object with dunder enter and dunder exit. The with statement calls enter, binds its return value to the as target, and guarantees exit runs afterward even if the body raises. Exit receives the exception details and can suppress the exception by returning True. The contextlib.contextmanager decorator lets you write the same thing as a generator with a single yield.

The protocol is two methods. Enter runs setup and returns whatever you want bound after the as keyword, and exit runs teardown. The important part is that exit runs whether the block finished cleanly or blew up, and it gets the exception type, value, and traceback so it can decide what to do. Returning True from exit swallows the exception, which I use sparingly because it hides failures. Most of the ones I write use contextlib.contextmanager, where the code before yield is setup, the code after is teardown, and I wrap the yield in try finally so cleanup still happens on an error. I built one at a fintech for database transactions that also stamped a request id onto the logging context, so every query inside the block was traceable and the id was cleared on the way out. Before that we were setting and clearing it by hand, and an early return in one handler leaked the id into the next request on the same worker.