Restore default implementation of __ne__ in mixins Set and Mapping

[serhiy-storchaka]

BPO	41416
Nosy	@rhettinger, @serhiy-storchaka
PRs	gh-85572: Remove references to nonexisting __ne__ methods #21628

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

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-07-28.06:34:53.290>
labels = ['type-bug', 'library', '3.10']
title = 'Restore default implementation of __ne__ in mixins Set and Mapping'
updated_at = <Date 2022-01-29.22:36:18.887>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2022-01-29.22:36:18.887>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-07-28.06:34:53.290>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 41416
keywords = ['patch']
message_count = 2.0
messages = ['374470', '374549']
nosy_count = 2.0
nosy_names = ['rhettinger', 'serhiy.storchaka']
pr_nums = ['21628']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41416'
versions = ['Python 3.10']

Linked PRs

• gh-85588: Restore the __ne__() method in Set and Mapping #141366

[serhiy-storchaka]

According to the documentation [1] abstract classes collections.abc.Set and collections.abc.Mapping provide mixin method __ne__. But implementations of __ne__ in these classes were removed in 3.4 (see bpo-21408), so the default implementation is inherited from object. The reason is that it works better with classes which override __ne__ to return non-boolean, e.g. NumPy, SymPy and sqlalchemy classes. Previously the != operator falled back to other__eq__(), now it falls back to other__ne__().

[1] https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes

But now we have a discrepancy between the decomentation and the code. According to the documentation, if we have a class which inherits from both Set and int, in that order, the __ne__ method should be inherited from class Set instead of class int. But currently it is inherited from int.

>>> import collections.abc
>>> class A(collections.abc.Set, int): pass
... 
>>> A.__ne__
<slot wrapper '__ne__' of 'int' objects>

One way to solve this -- remove __ne__ from lists of mixin methods (see bpo-41400). Other way -- add the __ne__ implementations which are identical to the default implementation but has preference in the inheritance. I prefer the latter.

[rhettinger]

If you want to add back the __ne__() method, that would be fine.

FWIW, "class A(collections.abc.Set, int)" seems pretty exotic to me. I wouldn't have expected that work out well.