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

[Martijn Pieters]

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 at python
components: Documentation, Library (Lib)
messages: 240060
nosy: docs at 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 <report at bugs.python.org>
<http://bugs.python.org/issue23864>
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