Guaranteed cleanup

The current code acquires the lock and then releases it, with a line in between that can fail. If it does, execution never reaches release, the lock stays held forever, and every call after that fails. Run the code and see for yourself.

with guarantees the cleanup. It calls __enter__ on the way in, puts whatever that returns into the variable after as, then calls __exit__ on the way out, whatever the reason, whether the code finished normally or raised an exception. That's how with open(...) works: you write the cleanup once in the class instead of writing try/finally everywhere the class is used.

The trap is that __exit__ receives the exception details, and if it returns True it hides the exception as though it never happened. So return nothing from it, and let the error carry on to whoever handles it. If you'd rather write a function than a class, there's a shorter way: @contextlib.contextmanager with yield inside try/finally. Forget the finally there and you're back to the same problem.