inspect.signature returns incorrect result on a @staticmethod and a @classmethod

[heckad]

Bug report

import inspect


def test_1():
    class FooStaticmethod:
        @staticmethod
        def __call__(cls, s, l, t):
            return t

    instance = FooStaticmethod()
    instance(1, 2, 3, 4) # Success call with 4 args

    assert list(inspect.signature(instance).parameters.keys()) == ['cls', 's', 'l', 't']


def test_2():
    class FooClassmethod:
        @classmethod
        def __call__(cls, s, l, t):
            return t

    instance = FooClassmethod()
    instance(1, 2, 3) # Success call with 4 args

    assert list(inspect.signature(instance).parameters.keys()) == ['s', 'l', 't']

def test_3():
    class FooStaticmethod:
        @staticmethod
        def __call__():
            return None

    instance = FooStaticmethod()
    instance()

    assert list(inspect.signature(instance).parameters.keys()) == []

Your environment

CPython versions tested on:

• Python 3.10.9 (tags/v3.10.9:1dd9be6, Dec 6 2022, 20:01:21) [MSC v.1934 64 bit (AMD64)] on win32
• Python 3.11.1 (tags/v3.11.1:a7a450f, Dec 6 2022, 19:58:39) [MSC v.1934 64 bit (AMD64)] on win32

Possible fix

The problem is that we are calling _signature_from_callable twice with skip_bound_arg=True.
A simple solution is to call _get_signature_of with skip_bound_arg=False on

cpython/Lib/inspect.py

Line 2610 in f02fa64

sig = _get_signature_of(call)

That fix the test_2 but not the test_1. I need to think about how to do better. I will be glad to your suggestions.

Р.S I would like to submit a pull request.

Linked PRs

• gh-101293: Fix inspect.signature behavior on __call__ decorated with staticmethod and classmethod decorators. #102564
• gh-101293: Fix support of custom callables and types in inspect.Signature.from_callable() #115530
• [3.11] gh-101293: Fix support of custom callables and types in inspect.Signature.from_callable() (GH-115530) #116197
• [3.12] gh-101293: Fix support of custom callables and types in inspect.Signature.from_callable() (GH-115530) #116198

[sobolevn]

I don't think that __call__ should ever be @staticmethod or @classmethod. It is designed to be an instance method. Do you have examples with other methods?

[heckad]

It is from pyparsing, Here is the test suite. I want to do this and got a bug in inspect module.

Is there a way to find out which descriptor wrapped a method?

[serhiy-storchaka]

__call__() can be an arbitrary callable or descriptor.

This is an interesting issue, and it is larger that what #102564 covers.

• It does not handle classmethod or statimethod decorated __call__() for types. It is more rare case, but __call__() can be overridden in metaclasses, and this case is supported.
• Standard type.__call__() calls __new__() and __init__(), which also can be not only pure Python functions.
• There are more types of callables and descriptors that can be used here: bound methods, partial(), and various builtin methods and descriptors.

#115530 is more general, and supports more different cases.

It does not support all cases. There is an explicit exclusion for builtin methods and descriptors in the current code, and I keep it for now. It can be significantly weakened (allowing to support more weird cases), but this can also break some Cython generated classes, so we should be careful. More general approach would ignore signatures like (*args, **kwargs), but it requires large changes.