ctypes/__init__.pyi uses TypeVar within ClassVar

[erictraut]

I just implemented a check in pyright to report an error if a TypeVar is used within a ClassVar declaration. PEP 526 specifically says:

Note that a ClassVar parameter cannot include any type variables, regardless of the level of nesting: ClassVar[T] and ClassVar[List[Set[T]]] are both invalid if T is a type variable.

A similar feature request was recently filed in the mypy issue tracker.

Here's the issue in the pyright issue tracker: microsoft/pyright#2567.

This check is highlighting a problem in the pointer class defined within the ctypes/__init__.pyi stub.

class pointer(Generic[_CT], _PointerLike, _CData):
    _type_: ClassVar[Type[_CT]]

I don't know enough about ctypes to suggest a proper fix.

[JelleZijlstra]

At runtime ctypes.pointer is a function that creates (!) a type when you call it, and returns an instance of that type. _type_ is indeed a ClassVar on that type, but probably we should just model it as a regular attribute.

Reference: https://github.com/python/cpython/blob/main/Modules/_ctypes/callproc.c#L1947. The POINTER function right above it is the one that creates the type.

[erictraut]

The Array class within the same file also exhibits this problem.

class Array(Generic[_CT], _CData):
    _type_: ClassVar[Type[_CT]]
    ...
``

[sobolevn]

I can make a PR to mypy to forbid this pattern (python/mypy#11538). Looks like we would need to fix typeshed first.

[sobolevn]

Mypy PR: python/mypy#11585