You are basically right that it is garbage collection which causes your generator instance to be cleaned up, and in this instance it is happening after “time.sleep” has been set to None. It could also happen in the order, which would explain why changing names changes the behaviour you see.

Why the interpreter has to set module globals to None I’ve never quite understood. Maybe it is to guarantee that modules involved in garbage cycles with __del__() can still be partly cleaned up, otherwise such a cycle could easily keep lots of modules alive via globals.

This is not documented very obviously either. The best I can find at the moment is the warning under <https://docs.python.org/2/reference/datamodel.html#object.__del__>, which says “when __del__() is invoked in response to a module being deleted, [globals may be] in the process of being torn down”. The same applies to generator cleanup code. Looking through Python code you will often find workarounds such as prefixing globals with an underscore or caching references in non-global locations.

My recommended workaround though is to avoid relying on garbage collection as much as possible, and instead use a try / finally or “with” block. Then you know exactly when your generator will be cleaned up:

ct = create_generator()
try:
    ct.next()
finally:
    ct.close()

from contextlib import closing
with closing(create_generator()) as ct:
    ct.next()