New submission from FHTMitchell <fhtmitch...@gmail.com>:
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.
----------
messages: 391705
nosy: FHTMitchell
priority: normal
severity: normal
status: open
title: Can't create generic NamedTuple as of py3.9
versions: Python 3.10, Python 3.11, Python 3.9
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue43923>
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