New submission from Martijn Pieters:
The collections.abc documentation implies that *any* of the container ABCs can
be used in an issubclass test against a class that implements all abstract
methods:
> These ABCs allow us to ask classes or instances if they provide particular
> functionality [...]
In reality this only applies to the "One Trick Ponies" (term from PEP 3119,
things like Container and Iterable, those classes with one or two methods). It
fails for the compound container ABCs:
>>> from collections.abc import Sequence, Container, Sized
>>> 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
...
>>> issubclass(MySequence, Container)
True
>>> issubclass(MySequence, Sized)
True
>>> issubclass(MySequence, Sequence)
False
That's because the One Trick Ponies implement a __subclasshook__ method that is
locked to the specific class and returns NotImplemented for subclasses; for
instance, the Iterable.__subclasshook__ implementation is:
@classmethod
def __subclasshook__(cls, C):
if cls is Iterable:
if any("__iter__" in B.__dict__ for B in C.__mro__):
return True
return NotImplemented
The compound container classes build on top of the One Trick Ponies, so the
class test will fail, NotImplemented is returned and the normal ABC tests for
base classes that have been explicitly registered continues, but this won't
include unregistered complete implementations.
Either the compound classes need their own __subclasshook__ implementations, or
the documentation needs to be updated to make it clear that without explicit
registrations the issubclass() (and isinstance()) tests only apply to the One
Trick Ponies.
----------
assignee: docs@python
components: Documentation, Library (Lib)
messages: 240060
nosy: docs@python, mjpieters
priority: normal
severity: normal
status: open
title: issubclass without registration only works for "one-trick pony"
collections ABCs.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue23864>
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