ctypes POINTER invalid incompatible type with None

With type checkers (PyCharm's, MyPy) I get the error:
mypy_test.py:6: error: Argument 1 to "POINTER" has incompatible type "None"; expected "type[Never]" [arg-type]

Using this simple example:

from ctypes import POINTER
void = None
POINTER(void)

POINTER(None) in ctypes is equivalent to c_void_p and is valid when ran. Minor issue but is technically a compatible type.

Is this change reasonable?

typeshed/stdlib/_ctypes.pyi

Lines 101 to 102 in dbd0d35


	def POINTER(type: type[_CT], /) -> type[_Pointer[_CT]]: ...

+@overload
+def POINTER(type: None, /) -> type[c_void_p]: ...
+@overload
def POINTER(type: type[_CT], /) -> type[_Pointer[_CT]]: ...

I'm no expert for ctypes, but I think c_void_p (without type) is the correct return type:

In [6]: POINTER(None) is c_void_p
Out[6]: True

ctypes.POINTER returns a pointer type for the given type, based on the private _pointer_type_cache mapping.

The private function _reset_cache is called when the module is loaded and clears that mapping, assigning None as the key and c_void_p as the value.

Therefore, I think it is reasonable that POINTER(None) returns type[c_void_p].

I recently dived into the ctypes implementation while maintaining a project that needed to be updated to accommodate recent changes made to ctypes. This is where I gained this insight.

I'm planning to submit a PR regarding this.

Fixes python#12736

I'm no expert for ctypes, but I think c_void_p (without type) is the correct return type:
In [6]: POINTER(None) is c_void_p
Out[6]: True

Come to think of it with fresh eyes. Doesn't that confirm the return is indeed the class to be instanciated? (represented by type[c_void_p])?

>>> import ctypes
>>> ctypes.POINTER(None) is ctypes.c_void_p
True
>>> isinstance(ctypes.POINTER(None)(), ctypes.c_void_p)
True

Let's do the same exercise with a simple type (int):

>>> int is int
True
>>> isinstance(int(), int)
True

I'm not sure where the confusion was, maybe I was also too sleepy ^^"

I'm not sure where the confusion was, maybe I was also too sleepy ^^"

You're completely right, of course. No idea why I was confused.

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ctypes POINTER invalid incompatible type with None #12736

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ctypes POINTER invalid incompatible type with None #12736

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junkmd commented on Oct 7, 2024

srittau commented on Oct 8, 2024

junkmd commented on Oct 8, 2024

Avasam commented on Oct 9, 2024

srittau commented on Oct 10, 2024

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