Wow: list of immediate subclasses?

[Michael Hudson]

"Andrew Dalke" <adalke at mindspring.com> writes:

> Carl Banks, responding to Roman Suzi:
> > __subclasses__ is a list of weak references, so the class can be
> > collected even though it's listed in __subclasses__.  For whatever
> > reason, class A isn't deleted by reference counts, but cyclic garbage
> > collection gets it.
> 
> But I want to know that reason.  Why are there any cycles?  After
> all, don't weak-references help break cycles?  And in my code
> before, why do I need to do the gc a few times to force the cycle
> to be broken?
> 
> >>> float.__subclasses__()
> []
> >>> class Spam(float): pass
> ...
> >>> float.__subclasses__()
> [<class '__main__.Spam'>]
> >>> del Spam
> >>> _ = None  # just in case ...

This doesn't do what you think it does.  The reference to Spam is in
__builtin__._; this only assigns to __main__._.

I don't know where the cycle is, off-hand.  I suspect I did once...

[...]
> It's a bit too mysterious for my liking.

I can manage not to care :-)

Cheers,
mwh

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