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

[Manuel Cerón]

Change by Manuel Cerón :


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

[Cheryl Sabella]

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

[Guido van Rossum]

Guido van Rossum  added the comment:

No, I consider this is a documentation problem. I don't recall why the docs say 
that (I don't even know if they still say that or whether Martijn misread 
them), but IMO this should not be changed.

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

[Géry]

Géry  added the comment:

Guido, could we add those missing __subclasshook__ for consistency?

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

[Saul Shanabrook]

Saul Shanabrook added the comment:

I have added a failing test to based on the first example, of a class that 
provides the necessary methods, but fails to be an instance of Sequence.

--
keywords: +patch
nosy: +saulshanabrook
Added file: http://bugs.python.org/file38935/patch.diff

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

[Saul Shanabrook]

Changes by Saul Shanabrook s.shanabr...@gmail.com:


Removed file: http://bugs.python.org/file38935/patch.diff

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

[Saul Shanabrook]

Changes by Saul Shanabrook s.shanabr...@gmail.com:


Added file: http://bugs.python.org/file38937/patch.diff

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

[Saul Shanabrook]

Changes by Saul Shanabrook s.shanabr...@gmail.com:


Removed file: http://bugs.python.org/file38937/patch.diff

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

[Saul Shanabrook]

Changes by Saul Shanabrook s.shanabr...@gmail.com:


Added file: http://bugs.python.org/file38938/patch.diff

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

[Jon Clements]

Changes by Jon Clements jon...@googlemail.com:


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

[Martijn Pieters]

Martijn Pieters added the comment:

I should have added the mixin methods for the Sequence implementation; the more 
complete demonstration is:

 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
... 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

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

[Antti Haapala]

Antti Haapala added the comment:

This does apply to all versions of Python from 2.6 up. Registering does work of 
course.

I believe the reason for not having the __subclasshook__ is the following 
sentence in PEP 3119: ABCs are intended to solve problems that don't have a 
good solution at all in Python 2, such as distinguishing between mappings and 
sequences.

This used to be worse in 3.3 because there if you ever inherit from `Sequence` 
you will always end up having `__dict__`, even if you just want `__slots__`.

(By the way, if Py2 documentation is fixed, it should also say that these ABCs 
are new as of 2.6, not since 2.4 like the rest of the collections module).

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

[eryksun]

eryksun added the comment:

Probably I'm overlooking something, but why isn't this hook defined 
cooperatively, with a terminating base class method that returns True? If the 
call chain progresses to the base, then all of the interfaces have been 
satisfied. Otherwise one of the bases returns NotImplemented. If it's 
implemented cooperatively, then the `cls is Iterable` check can be removed, 
because it returns super().__subclasshook__(C) instead of True.

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