Behaviour of __subclasses__ when classes are deleted

Question

Edit: Generalised the question due to NPE\'s comment.

In a Python 2.7.3 interactive session:

>>> class Foo(object):
...     pass
... 
>&g         


        

Answer 1

See this thread. It would seem that what happens is the class's __mro__ attribute stores a reference to itself, creating a reference cycle. You can force a full gc run which will detect the cycle and delete the object:

>>> class Foo(object): pass
>>> class Bar(Foo): pass
>>> import gc
>>> del Bar
>>> gc.collect()
3
>>> Foo.__subclasses__()
[]

Alternatively, if you enter other commands for a while, the gc will run on its own and collect the cycle.

Note that you have to be a bit careful when testing this interactively, because the interactive interpreter stores a reference to the most recently returned value in the "last value" variable _. If you explicitly look at the subclass list and then immediately try to collect, it won't work, because the _ variable will hold a list with a strong reference to the class.