typing.NamedTuple does not support mixins #80698

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1. rectalogic commented on Apr 3, 2019

   

   rectalogicmannequin

   on Apr 3, 2019

   MannequinAuthor

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   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'
2. rectalogic

   added

   3.7 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 3, 2019
3. ilevkivskyi commented on Apr 6, 2019

   

   ilevkivskyi

   on Apr 6, 2019

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   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.
4. serhiy-storchaka commented on Apr 4, 2020

   

   serhiy-storchaka

   on Apr 4, 2020

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   New changeset a94e627 by Serhiy Storchaka in branch 'master':
   bpo-36517: Raise error on multiple inheritance with NamedTuple (GH-19363)
   a94e627
5. bentheiii commented on Feb 9, 2021

   

   bentheiiimannequin

   on Feb 9, 2021

   Mannequin

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   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.
6. ezio-melotti

   transferred this issue fromon Apr 10, 2022
7. AlexWaygood

   added

   topic-typing

   and removed

   3.7 (EOL)end of lifeend of life

   on Apr 13, 2022
8. AlexWaygood commented on Apr 10, 2023

   

   AlexWaygood

   on Apr 10, 2023

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   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.
9. AlexWaygood

   closed this as completedon Apr 10, 2023