[docs] [issue23864] issubclass without registration only works for "one-trick pony" collections ABCs.

[Cheryl Sabella]

Cheryl Sabella <cheryl.sabella at gmail.com> added the comment:

This isn't meant as a comment from any previous posts.  It's simply meant to correct a statement (based on new information in the past 2 years) from the original post.

Since this original report, some ABCs that are not "One Trick Ponies" have been added which implement  __subclasshook__.  `Collection` is one of those, so using the original example:

>>> from collections.abc import Sequence, Container, Sized, Collection
>>> class MySequence(object):
...     def __contains__(self, item): pass
...     def __len__(self): pass
...     def __iter__(self): pass
...     def __getitem__(self, index): pass
...     def __len__(self): pass
...     def __reversed__(self): pass
...     def index(self, item): pass
...     def count(self, item): pass
... 
>>> issubclass(MySequence, Container)
True
>>> issubclass(MySequence, Sized)
True
>>> issubclass(MySequence, Sequence)
False
>>> issubclass(MySequence, Collection)
True

Collection is not a "One Trick Pony" because it is used for Sized, Iterable Containers.

Generator, Coroutine, and ASyncGenerator are also not "One Trick Ponies" (although they are defined under that section in _collections_abc.py).

Again, for reference, the definition of One Trick Pony from PEP3119 is:
These abstract classes represent single methods like __iter__ or __len__.

If only One Trick Ponies implemented __subclasshook__, then the original documentation issue:
> These ABCs allow us to ask classes or instances if they provide particular functionality, for example:

maybe could have been changed to:
> These ABCs allow us to ask classes or instances if they provide singular functionality, for example:

But, that's not really correct anymore.

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nosy: +cheryl.sabella

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue23864>
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