PEP 695 syntax doesn't work for generic NamedTuples

[AlexWaygood]

Since Python 3.11, defining generic NamedTuples has been legal, like so:

>>> from typing import *
>>> T = TypeVar("T")
>>> class Foo(NamedTuple, Generic[T]):
... x: T
>>>

However, using the new PEP-695 syntax for generic NamedTuples fails with a slightly inexplicable error message:

>>> from typing import *
>>> class Foo[T](NamedTuple):
... x: T
...
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 1, in <generic parameters of Foo>
 File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 2707, in __new__
 setattr(nm_tpl, key, ns[key])
AttributeError: attribute '__type_params__' of 'type' objects is not writable

Using PEP-695 syntax for generic TypedDicts seems to work fine, but isn't tested anywhere in test_typing.py, as far as I can tell:

>>> class Bar[T](TypedDict):
... x: T
...
>>> Bar.__type_params__
(T,)
>>> Bar.__orig_bases__
(<function TypedDict at 0x0000017E9A17D9D0>, typing.Generic[T])
>>> Bar.__mro__
(<class '__main__.Bar'>, <class 'typing.Generic'>, <class 'dict'>, <class 'object'>)

Cc. @JelleZijlstra

[AlexWaygood]

I guess we need to add __type_params__ to one of these two sets here?

cpython/Lib/typing.py

Lines 2668 to 2673 in cfa517d

	# attributes prohibited to set in NamedTuple class syntax
	_prohibited = frozenset({'__new__', '__init__', '__slots__', '__getnewargs__',
	'_fields', '_field_defaults',
	'_make', '_replace', '_asdict', '_source'})
	_special = frozenset({'__module__', '__name__', '__annotations__'})

[JelleZijlstra]

Thanks! I'll take a look tonight (though feel free to propose a solution yourself if you can get to it earlier). Adding to _special feels like it's not enough, because that will just skip the assignment, and we still want the __type_params__ to end up on the generated class somehow.

[AlexWaygood]

Thanks! I'll take a look tonight (though feel free to propose a solution yourself if you can get to it earlier). Adding to _special feels like it's not enough, because that will just skip the assignment, and we still want the __type_params__ to end up on the generated class somehow.

Ah. I agree that the generated class should have the correct __type_params__ value. And, I didn't realise that it wasn't possible to set the __type_params__ field on class objects. Maybe we should allow setting it on class objects, the same as you propose for function objects in #104600? I don't see a strong reason for it to be read-only; this case would benefit from it; and in general I feel like you should be able to mutate most fields on class objects

[JelleZijlstra]

Yes, it probably should be writable; I thought it already was. I can make that change.

[JelleZijlstra]

Fixed in #104634