typing.NamedTuple does not support mixins

[rectalogic]

BPO	36517
Nosy	@rhettinger, @serhiy-storchaka, @ilevkivskyi, @bentheiii
PRs	bpo-36517: Raise error on multiple inheritance with NamedTuple #19363

^{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 2019-04-03.13:07:30.022>
labels = ['3.7', 'type-bug', 'library']
title = 'typing.NamedTuple does not support mixins'
updated_at = <Date 2021-02-09.09:57:06.552>
user = 'https://bugs.python.org/rectalogic'

bugs.python.org fields:

activity = <Date 2021-02-09.09:57:06.552>
actor = 'avrahami.ben'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-04-03.13:07:30.022>
creator = 'rectalogic'
dependencies = []
files = []
hgrepos = []
issue_num = 36517
keywords = ['patch']
message_count = 4.0
messages = ['339390', '339546', '365771', '386701']
nosy_count = 5.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'levkivskyi', 'rectalogic', 'avrahami.ben']
pr_nums = ['19363']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36517'
versions = ['Python 3.7']

[rectalogic]

Subclassing typing.NamedTuple an inheriting from a mixin class does not work. It does work for collections.namedtuple, and can be worked around by modifying typing.NamedTupleMeta:

>>> import collections
>>> import typing
>>>
>>>
>>> class Mixin:
...     def mixin(self):
...         return "mixin"
...
>>>
>>> class CollectionsNamedTuple(Mixin, collections.namedtuple('CollectionsNamedTuple', [
...     "a",
...     "b",
... ])):
...     pass
...
>>>
>>> class TypingNamedTuple(Mixin, typing.NamedTuple):
...     a: str
...     b: str
...
>>>
>>> class NamedTupleMeta(typing.NamedTupleMeta):
...     def __new__(cls, typename, bases, ns):
...         cls_obj = super().__new__(cls, typename + '_nm_base', bases, ns)
...         bases = bases + (cls_obj,)
...         return type(typename, bases, {})
...
>>>
>>> class FixedTypingNamedTuple(Mixin, metaclass=NamedTupleMeta):
...     a: str
...     b: str
...
>>>
>>> cnt = CollectionsNamedTuple("av", "bv")
>>> tnt = TypingNamedTuple("av", "bv")
>>> ftnt = FixedTypingNamedTuple("av", "bv")
>>>
>>> cnt.mixin()
'mixin'
>>> ftnt.mixin()
'mixin'
>>> tnt.mixin()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'TypingNamedTuple' object has no attribute 'mixin'

[ilevkivskyi]

Hm, it looks like we can support this. I however don't think the proposed "patch" is the right way to fix it, since this makes the implementation with and without a mixin quite different.

Also I am not sure I will have time to work on this in the nearest month.

[serhiy-storchaka]

New changeset a94e627 by Serhiy Storchaka in branch 'master':
bpo-36517: Raise error on multiple inheritance with NamedTuple (GH-19363)
a94e627

[bentheiii]

The patch PR blocks out a useful idiom: generic Nametuple

>>> class LLNode(NamedTuple, Generic[T]):
...     value :T
...     next: Optional[LLNode[T]]

I put forward that, at the least, NamedTuple should accept do-nothing bases like Generic.

[AlexWaygood]

Generic NamedTuples are supported on 3.11+, and the decision has been taken to prohibit mixins in other situations. I don't think we'll be changing that now. Closing as completed.