how to type hint ctypes.POINTER(ctypes.c_int)

Question

Here is an MRE you can run on mypy Playground:

import ctypes
import numpy as np  # type: ignore # (no stubs for numpy)

def np_to_c(arr: np.ndarray) -> tuple[ctypes.POINTER(ctypes.c_int), ctypes.c_int]:
    return arr.ctypes.data_as(ctypes.POINTER(ctypes.c_int)), ctypes.c_int(len(arr))

This type hinting is wrong and according to mypy:

main.py:4: error: Invalid type comment or annotation  [valid-type]
main.py:4: note: Suggestion: use ctypes.POINTER[...] instead of ctypes.POINTER(...)
Found 1 error in 1 file (checked 1 source file)

but when I do use ctypes.POINTER[...] instead of ctypes.POINTER(...) (cf screenshot below), I get:

main.py:4: error: Function "ctypes.POINTER" is not valid as a type  [valid-type]
main.py:4: note: Perhaps you need "Callable[...]" or a callback protocol?
Found 1 error in 1 file (checked 1 source file)

Answer 1

You could use a different definition specifically when running type checking:

import ctypes
from typing import TYPE_CHECKING
import numpy as np  # type: ignore # (no stubs for numpy)

if TYPE_CHECKING:
    IntPointer = ctypes._Pointer[ctypes.c_int]
else:
    IntPointer = ctypes.POINTER(ctypes.c_int)

def np_to_c(arr: np.ndarray) -> tuple[IntPointer, ctypes.c_int]:
    return arr.ctypes.data_as(IntPointer), ctypes.c_int(len(arr))

Try me on mypy-play