Can't create generic NamedTuple as of py3.9

[FHTMitchell]

BPO	43923
Nosy	@gvanrossum, @rhettinger, @ericvsmith, @serhiy-storchaka, @graingert, @ilevkivskyi, @dlukes, @JelleZijlstra, @FHTMitchell, @sobolevn, @Fidget-Spinner, @AlexWaygood, @juliusgeo
PRs	bpo-43923: Revert "bpo-40185: Refactor typing.NamedTuple (GH-19371)" #31679 bpo-43923: Allow NamedTuple multiple inheritance #31779 gh-116241: Add support of multiple inheritance with typing.NamedTuple #31781

^{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 = None
created_at = <Date 2021-04-23.16:22:56.566>
labels = ['type-bug', '3.9', '3.10', '3.11']
title = "Can't create generic NamedTuple as of py3.9"
updated_at = <Date 2022-03-10.21:38:13.215>
user = 'https://github.com/FHTMitchell'

bugs.python.org fields:

activity = <Date 2022-03-10.21:38:13.215>
actor = 'behackett'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-04-23.16:22:56.566>
creator = 'FHTMitchell'
dependencies = []
files = []
hgrepos = []
issue_num = 43923
keywords = ['patch']
message_count = 25.0
messages = ['391705', '406083', '406124', '414541', '414557', '414558', '414559', '414563', '414567', '414568', '414570', '414584', '414585', '414589', '414592', '414593', '414705', '414709', '414754', '414791', '414792', '414815', '414816', '414817', '414830']
nosy_count = 16.0
nosy_names = ['gvanrossum', 'rhettinger', 'eric.smith', 'python-dev', 'serhiy.storchaka', 'graingert', 'levkivskyi', 'dlukes', 'JelleZijlstra', 'behackett', 'FHTMitchell', 'Steven Silvester', 'sobolevn', 'kj', 'AlexWaygood', 'juliusgeo']
pr_nums = ['31679', '31779', '31781']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43923'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[FHTMitchell]

As of python 3.9, you now can't have multiple inheritance with typing.NamedTuple subclasses. This seems sensible, until you realise that typing.Generic works via inheritance. This fails whether or not from __future__ import annotations is enabled.

example:

class Group(NamedTuple, Generic[T]):
     key: T
     group: List[T]
 
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-fc238c4826d7> in <module>
----> 1 class Group(NamedTuple, Generic[T]):
      2     key: T
      3     group: List[T]
      4 

~/.conda/envs/py39/lib/python3.9/typing.py in _namedtuple_mro_entries(bases)
   1818 def _namedtuple_mro_entries(bases):
   1819     if len(bases) > 1:
-> 1820         raise TypeError("Multiple inheritance with NamedTuple is not supported")
   1821     assert bases[0] is NamedTuple
   1822     return (_NamedTuple,)

TypeError: Multiple inheritance with NamedTuple is not supported

This worked fine in python 3.7 and 3.8 and as I understand it was one of the motivating cases for PEP-560.

The change was made as part of bpo-40185: Refactor typing.NamedTuple. Whilst the obvious alternative is "use dataclasses", they don't have the same runtime properties or implications as namedtuples.

[dlukes]

This is unfortunate, especially since it used to work... Going forward, is the intention not to support this use case? Or is it possible that support for generic NamedTuples will be re-added in the future?

[rhettinger]

+1 for reverting this change and restoring the previous behavior.

[gvanrossum]

Couldn't there be a subtler solution than rolling back #63570?

[JelleZijlstra]

Was this ever documented to work? We have now disallowed this behavior in 3.9 and 3.10 with few complaints, so it doesn't seem that important to restore it. Also, static type checkers generally disallow generic NamedTuples.

[gvanrossum]

Mypy seems to allow this:

from typing import NamedTuple, TypeVar, Generic, List, Tuple

T = TypeVar("T")

class New(NamedTuple, Generic[T]):
    x: List[T]
    y: Tuple[T, T]

It's true that pyright doesn't, but maybe that's because it doesn't work in 3.9-3.10?

[JelleZijlstra]

It doesn't really. If you do x = New([1], (2, 3)) you get:

main.py:11: error: List item 0 has incompatible type "int"; expected "T"
main.py:11: error: Argument 2 to "New" has incompatible type "Tuple[int, int]"; expected "Tuple[T, T]"

https://mypy-play.net/?mypy=latest&python=3.10&gist=a13c7a33c55a3aeee95324d46cd03ffd