[New-bugs-announce] [issue36517] typing.NamedTuple does not support mixins

[Andrew Wason]

New submission from Andrew Wason <rectalogic at rectalogic.com>:

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'

----------
components: Library (Lib)
messages: 339390
nosy: rectalogic
priority: normal
severity: normal
status: open
title: typing.NamedTuple does not support mixins
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36517>
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