How To Create A Custom Generator Class That Is Correctly Garbage Collected

I'm trying to write a class in Python that behaves as a generator object, particularly in that when it's garbage collected .close() is called on it. That's important because it mea

Solution 1:

PEP342, states:

[generator].__del__() is a wrapper for [generator].close(). This will be called when the generator object is garbage-collected ...

The Generator class in collections.abc does not implement __del__, and neither do its superclasses or metaclass.

Adding this implementation of __del__ to the class in the question results in the lock being freed:

class CustomGeneratorClass(Generator):

    ...

    def __del__(self):
        self.close() 

Output:

Generator Class Initialised: I grabbed a lock
Recieved  0
Recieved  1
Recieved  2
Recieved  3
Exception Thrown in Generator: I let go of the lock
Finished: Lock was free

Caveat:

I'm not experienced with the intricacies of object finalisation in Python, so this suggestion should be examined critically, and tested to destruction. In particular, the warnings about __del__ in the language reference should be considered.

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A higher-level solution would be to run the generator in a context manager

with contextlib.closing(CustomGeneratorClass(100, lock)):
    # do stuff

but this is cumbersome, and relies on users of the code remembering to do it.