stubgen and Cython modules

Hello,

I'm trying to use mypy with a Cython module (via stubgen).

Example Cython module (test.pyx):

import typing

def f(path: str, a: int = 0, b: bool = True) -> typing.List[str]:
    return []

cdef class MyClass(object):
    def __init__(self, name: str = None):
        self.name = name

    def run(self, action: str) -> None:
        pass

Compile this module: cythonize -b test.pyx
The output will be a binary extension module (test..pyd/test..so).
Run stubgen on this via stubgen -m test.

In general, there's two ways in which Cython can expose the function signature:

• #cython: embedsignature=True: Cython will embed the signature in the docstring. This has some issues in Cython, see cython/cython#3150.
• #cython: binding=True: Cython will use a custom function object (not a PyCFunction) that has the relevant attributes set -- this allows using inspect.signature(). As of Cython 3, this option is enabled by default, and I'll only consider this option in the following.

Both f and MyClass.run are of type cython_function_or_method, which is neither a Python builtin function, nor a PyCFunction.
Instead, it's effectively a custom callable that has most of the attributes expected for a function:

>>> dir(test.f)
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__vectorcalloffset__', '_is_coroutine', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name']
>>> test.f.__annotations__
{'path': 'str', 'a': 'int', 'b': 'bool', 'return': 'typing.List[str]'}
>>> inspect.signature(test.MyClass.run)
<Signature (self, action: 'str') -> 'None'>

However stubgen fails to correctly handle this custom callable, and ends creating:

import _cython_3_1_3

__test__: dict
f: _cython_3_1_3.cython_function_or_method

class MyClass:
    def __init__(self, *args, **kwargs) -> None: ...
    def run(self, *args, **kwargs): ...
    def __reduce__(self): ...
    def __reduce_cython__(self, *args, **kwargs): ...
    def __setstate_cython__(self, *args, **kwargs): ...

まず、Cython モジュールと stubgen -m test を使って例を再現し、次に stubgen が cython_function_or_method オブジェクトと __annotations__ および inspect.signature() を通じて公開されるシグネチャをどのように処理するかを追跡します。f と MyClass.run の示されたシグネチャが、汎用的な Callable 引数を使うのではなく、生成されたスタブに保持されれば完了です。