[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions:  -Python 3.6

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:


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


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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:


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


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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Change by Bar Harel :


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pull_requests: +17142
pull_request: https://github.com/python/cpython/pull/17685

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Change by Bar Harel :


--
pull_requests: +17141
pull_request: https://github.com/python/cpython/pull/17684

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:


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


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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Brett Cannon]

Change by Brett Cannon :


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nosy:  -brett.cannon

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Bar Harel  added the comment:

Ready for merge

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Bar Harel  added the comment:

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.

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +xtreak

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Ammar Askar]

Ammar Askar  added the comment:

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

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nosy: +ammar2

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

> 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.

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Brett Cannon]

Brett Cannon  added the comment:

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?

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nosy: +levkivskyi

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Bar Harel  added the comment:

Done.

On Fri, Nov 22, 2019, 12:23 PM Bar Harel  wrote:

>
> Change by Bar Harel :
>
>
> --
> keywords: +patch
> pull_requests: +16820
> stage: resolved -> patch review
> pull_request: https://github.com/python/cpython/pull/17336
>
> ___
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> 
> ___
>

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Change by Bar Harel :


--
keywords: +patch
pull_requests: +16820
stage: resolved -> patch review
pull_request: https://github.com/python/cpython/pull/17336

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Brett Cannon]

Brett Cannon  added the comment:

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

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resolution: not a bug -> 
status: closed -> open

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

Bar Harel  added the comment:

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

On Thu, Nov 21, 2019, 9:45 PM Brett Cannon  wrote:

>
> Brett Cannon  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
>
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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Brett Cannon]

Brett Cannon  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?

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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Batuhan]

Change by Batuhan :


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nosy: +brett.cannon

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[issue38878] os.PathLike subclasshook causes subclass checks true on abstract implementation

[Bar Harel]

New submission from Bar Harel :

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.

--
components: Library (Lib)
messages: 357174
nosy: bar.harel
priority: normal
severity: normal
status: open
title: os.PathLike subclasshook causes subclass checks true on abstract 
implementation
type: behavior
versions: Python 3.6, Python 3.7, Python 3.8, Python 3.9

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