Quick and small fix.

os.PathLike.__subclasshook__ does not check if cls is PathLike as abstract classes should.

This in turn causes this bug:

    class A(PathLike):
        pass

    class B:
        def __fspath__(self):
            pass

    assert issubclass(B, A)

I will fix the bug later today and push a patch over to python/cpython on GitHub.

I can't reproduce in Python 3.8.0:

>>> import os
>>> class A(os.PathLike): pass
...
>>> class B:
...     def __fspath__(self): pass
...
>>> issubclass(B, A)
True

Did you check against an older version of Python?

Hey Brett, that's exactly the bug. It's supposed to be False ofc.

On Thu, Nov 21, 2019, 9:45 PM Brett Cannon <report@bugs.python.org> wrote:

>
> Brett Cannon <brett@python.org> added the comment:
>
> I can't reproduce in Python 3.8.0:
>
> >>> import os
> >>> class A(os.PathLike): pass
> ...
> >>> class B:
> ...     def __fspath__(self): pass
> ...
> >>> issubclass(B, A)
> True
>
> Did you check against an older version of Python?
>
> ----------
> resolution:  -> not a bug
> stage:  -> resolved
> status: open -> closed
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue38878>
> _______________________________________
>

Ah, your `assert` call threw me since it does succeed so it isn't acting as a test case.

Done.

On Fri, Nov 22, 2019, 12:23 PM Bar Harel <report@bugs.python.org> wrote:

>
> Change by Bar Harel <bzvi7919@gmail.com>:
>
>
> ----------
> keywords: +patch
> pull_requests: +16820
> stage: resolved -> patch review
> pull_request: https://github.com/python/cpython/pull/17336
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue38878>
> _______________________________________
>

I just realized one problem with this is it explicitly requires subclassing the ABC while os.PathLike is supposed to represent a protocol (before typing.Protocol was a thing).

So why is it bad that in the example class B is considered a "subclass" of os.PathLike by implementing the protocol? Since it implements the expected protocol what exactly is being lost by not checking for an explicit registration or subclass?

> So why is it bad that in the example class B is considered a "subclass" of os.PathLike by implementing the protocol?

This is not bad, my understanding of the problem is that currently B is considered a subclass of A, while the latter should not be structural.

To give an analogy with PEP 544 (sorry, this is my favourite one :-)) consider this:

class P(Protocol):
    def some(self): ...

class C:
    def some(self): ...

Here C is obviously a "subclass" of P, but:

class Bad(P):  # <- this is _no_ a protocol, just a nominal class
    pass       # explicitly subclassing P

class Good(P, Protocol):  # <- this is a subprotocol that
    pass                  # happened to be identical to P

So here C is a "subclass" of Good, but not a "subclass" of Bad.

Just for reference/existing behavior:

>>> class A(collections.abc.Iterable): pass
...
>>> class B:
...   def __iter__(): pass
...
>>> issubclass(B, A)
False
>>> issubclass(B, collections.abc.Iterable)
True
>>> issubclass(A, collections.abc.Iterable)
True

A better example is this:

    class A(PathLike):
        def foo(self):
            """For all your foo needs"""

    class B:
        def __fspath__(self):
            pass

    issubclass(B, A) == True
    A().foo() # Yay, I Foo'd.
    B().foo() # oh barnacles, I should have stayed at home.

Ready for merge

New changeset eae87e3e4e0cb9a0ce10c2e101acb6995d79e09c by Ivan Levkivskyi (Bar Harel) in branch 'master':
bpo-38878: Fix os.PathLike __subclasshook__ (GH-17336)
https://github.com/python/cpython/commit/eae87e3e4e0cb9a0ce10c2e101acb6995d79e09c

New changeset 0846e5d4603434c2bbf8a528677cf1ff3fe29b95 by Ivan Levkivskyi (Bar Harel) in branch '3.8':
[3.8] bpo-38878: Fix os.PathLike __subclasshook__ (GH-17336) (GH-17684)
https://github.com/python/cpython/commit/0846e5d4603434c2bbf8a528677cf1ff3fe29b95

New changeset 59d06b987db34cde8783e265709366d244c9e35b by Ivan Levkivskyi (Bar Harel) in branch '3.7':
[3.7] bpo-38878: Fix os.PathLike __subclasshook__ (GH-17336) (GH-17685)
https://github.com/python/cpython/commit/59d06b987db34cde8783e265709366d244c9e35b