gh-96127: Fix inspect.signature call on mocks

[sobolevn]

Before:

>>> import inspect
>>> from unittest.mock import AsyncMock
>>> inspect.signature(AsyncMock())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/karthikeyan/stuff/python/cpython/Lib/inspect.py", line 3272, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/karthikeyan/stuff/python/cpython/Lib/inspect.py", line 3020, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/karthikeyan/stuff/python/cpython/Lib/inspect.py", line 2507, in _signature_from_callable
    return _signature_from_function(sigcls, obj,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/karthikeyan/stuff/python/cpython/Lib/inspect.py", line 2351, in _signature_from_function
    positional = arg_names[:pos_count]
                 ~~~~~~~~~^^^^^^^^^^^^
TypeError: 'Mock' object is not subscriptable
>>> 

After:

>>> from inspect import signature
>>> from unittest.mock import AsyncMock
>>> 
>>> mock = AsyncMock()
>>> signature(mock)
<Signature (*args, **kwargs)>

• Issue: Calling inspect.signature with an AsyncMock instance raises an exception #96127

Related #94050

[sobolevn]

Re-triggering CI.

[graingert]

I have a similar PR somewhere about this, but I'm on mobile so it's tricky to find

[graingert]

#94962 is this related?

[sobolevn]

They are not related as far as I can tell :)

Your PR does not fix this exact problem:

gh pr checkout 94962

Python 3.12.0a0 (heads/main-dirty:a36235d5c7, Sep 11 2022, 12:23:24) [Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import inspect
>>> from unittest.mock import AsyncMock
>>> inspect.signature(AsyncMock())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sobolev/Desktop/cpython/Lib/inspect.py", line 3272, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/Lib/inspect.py", line 3020, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/Lib/inspect.py", line 2507, in _signature_from_callable
    return _signature_from_function(sigcls, obj,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/Lib/inspect.py", line 2351, in _signature_from_function
    positional = arg_names[:pos_count]
                 ~~~~~~~~~^^^^^^^^^^^^
TypeError: 'Mock' object is not subscriptable

[cjw296]

@sobolevn - could you rebase and resolve the conflicts? I think this is then good to land after another quick eyeball from a reviewer...

[sobolevn]

Sure!

1. I've checked that this exception still happens on main with all the new changes (it does)
2. I've rebased my changes on top of main
3. Force pushed! 🎉

I hope we can land this now!