issubclass on two different subclasses of abstract base class like os.PathLike returns unexpected value on early versions of Py3.7 and Py3.8

[mobiusklein]

BPO	47055
Nosy	@ericvsmith, @mobiusklein

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2022-03-20.23:20:36.269>
created_at = <Date 2022-03-18.02:52:35.204>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = '`issubclass` on two different subclasses of abstract base class like `os.PathLike` returns unexpected value on early versions of Py3.7 and Py3.8'
updated_at = <Date 2022-03-20.23:20:36.267>
user = 'https://github.com/mobiusklein'

bugs.python.org fields:

activity = <Date 2022-03-20.23:20:36.267>
actor = 'mobiusklein'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-20.23:20:36.269>
closer = 'mobiusklein'
components = ['Library (Lib)']
creation = <Date 2022-03-18.02:52:35.204>
creator = 'mobiusklein'
dependencies = []
files = []
hgrepos = []
issue_num = 47055
keywords = []
message_count = 3.0
messages = ['415467', '415494', '415642']
nosy_count = 2.0
nosy_names = ['eric.smith', 'mobiusklein']
pr_nums = []
priority = 'normal'
resolution = 'works for me'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue47055'
versions = ['Python 3.7', 'Python 3.8']

[mobiusklein]

The following code looks correct:

import os

class A(os.PathLike):
    pass

class B(os.PathLike):
    pass

assert issubclass(A, os.PathLike) # direct inheritance relationship
assert issubclass(B, os.PathLike) # direct inheritance relationship
assert not issubclass(A, B) # A is not derived from B or vice-versa

On Python 3.7.0-3.7.6 and 3.8.0-3.8.1 the third assertion fails. The expected behavior was restored in 3.7.7 and 3.8.2. This seems to be around when the internals of abc were translated to C.

Given that the behavior was fixed, but a bug fix matching this behavior wasn't listed in the changelog, the only resolution is to document that this behavior may be extant in older but still supported minor versions of Python. Where should that text be placed?

[ericvsmith]

Since those releases are no longer supported, I don't think there's any place you could put this that would be seen. And we wouldn't want to put a note in a current release about a bug in a non-supported version that was fixed in another non-supported version.

[mobiusklein]

Understood. It's probably good practice to keep the interpreter version up to date in any case when debugging.