What __enter__ and __exit__ do

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Implementing __enter__ and __exit__ makes your class a context manager, usable with the with statement. __enter__ receives the instance and can return a value bound to the as variable: with open('f') as fh. __exit__ receives the exception type, value, and traceback if one occurred. Returning True from __exit__ suppresses the exception; returning None or False lets it propagate. This protocol is what makes with guarantee cleanup - the same guarantee that finally gives you, but packaged reusably.