[issue2521] ABC caches should use weak refs

[Daniel Stutzbach]

Daniel Stutzbach <daniel at stutzbachenterprises.com> added the comment:

You can't register an old-style class, but many ABCs support duck-typing by implementing __subclasshook__.  ABCMeta caches those results and stores a reference to old-style classes, sometimes in _abc_cache and sometimes in _abc_negative_cache.  It can't simply return False.

I guess that leaves two options:

1) Make old-style classes weak-referenceable and cache the results of __subclasshook__.
2) Refuse to cache old-style classes and call __subclasshook__ every time for old-style classes.

Python 2.7a4+ (trunk:79493, Mar 30 2010, 19:19:13)
>>> class old_iterable_class:
...   def __iter__(self):
...     pass
...
>>> import collections
>>> issubclass(old_iterable_class, collections.Iterable)
True

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<http://bugs.python.org/issue2521>
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