Calling inspect.signature with an AsyncMock instance raises an exception

[yvan-the-barbarian]

Bug report

Starting in Python 3.10.6, the following code raises an exception:

from inspect import signature
from unittest.mock import AsyncMock

mock = AsyncMock()
signature(mock)

• No exception is raised when Mock is used.
• The exception does not occur with Python 3.10.5.

Your environment

• CPython versions tested on: 3.10.4, 3.10.5, 3.10.6 (Only occurs in 3.10.6)
• Operating system and architecture: Windows 10, Linux (Official Docker Image)

Linked PRs

• gh-96127: Fix inspect.signature call on mocks #96335
• [3.11] gh-96127: Fix inspect.signature call on mocks (#96335) #101646
• [3.10] gh-96127: Fix inspect.signature call on mocks (#96335) #101647
• [3.10] Fix inspect.signature call on mocks (#96335) (gh-96127) #108652

[graingert]

Related? #94924

[tirkarthi]

This could be due to commit 4261b6b

(myenv) ➜  cpython git:(main) ./python
Python 3.12.0a0 (heads/main:575f8880bf, Aug 23 2022, 19:59:51) [GCC 9.4.0] on linux
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 "/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
>>> 
(myenv) ➜  cpython git:(main) git log Lib/unittest/mock.py 
(myenv) ➜  cpython git:(main) git checkout 4261b6bffc0b8bb5c6d4d80578a81b7520f4aefc~1 Lib/unittest/mock.py
Updated 1 path from ec0ed6ea8a
(myenv) ➜  cpython git:(main) ✗ ./python                                                                    
Python 3.12.0a0 (heads/main:575f8880bf, Aug 23 2022, 19:59:51) [GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import inspect
>>> from unittest.mock import AsyncMock
>>> inspect.signature(AsyncMock())
<Signature (*args, **kwargs)>

[sobolevn]

@graingert yes, your link was very useful. Please, take a look at my proposed soultion :)

[cjw296]

@sobolevn - where is your proposed solution?

[AlexWaygood]

@sobolevn - where is your proposed solution?

• gh-96127: Fix inspect.signature call on mocks #96335

[hauntsaninja]

Looks like this has been fixed, thanks all!

[mdczaplicki]

I'm on 3.11.1 and this is happening.

    def _signature_from_function(cls, func, skip_bound_arg=True,
                                 globals=None, locals=None, eval_str=False):
        """Private helper: constructs Signature for the given python function."""
    
        is_duck_function = False
        if not isfunction(func):
            if _signature_is_functionlike(func):
                is_duck_function = True
            else:
                # If it's not a pure Python function, and not a duck type
                # of pure function:
                raise TypeError('{!r} is not a Python function'.format(func))
    
        s = getattr(func, "__text_signature__", None)
        if s:
            return _signature_fromstr(cls, func, s, skip_bound_arg)
    
        Parameter = cls._parameter_cls
    
        # Parameter information.
        func_code = func.__code__
        pos_count = func_code.co_argcount
        arg_names = func_code.co_varnames
        posonly_count = func_code.co_posonlyargcount
>       positional = arg_names[:pos_count]
E       TypeError: 'Mock' object is not subscriptable

[tirkarthi]

This commit is present on main branch and 3.12 latest alpha. I guess this is not backported.

[mdczaplicki]

Workaround:

from inspect import signature
from unittest.mock import AsyncMock, MagicMock

temp_mock = AsyncMock()
mock = MagicMock(return_value=temp_mock())
signature(mock)

[arhadthedev]

@sobolevn should the PR be backported to 3.11?

[sobolevn]

Automatic backport has failed :(
I don't have time right now to do it manually, please help me!

[arhadthedev]

Got it, will start this in a couple of hours.

[arhadthedev]

@sobolevn ping?

[arhadthedev]

@mdczaplicki @tirkarthi The backporting is done. The fix will be available in 3.11.3 and 3.10.11 this April.

[arhadthedev]

Fixed by gh-96335.

[AlexWaygood]

The fix will be available in 3.11.3 and 3.10.11 this April.

@arhadthedev where is the 3.10 backport?

[arhadthedev]

I confused it with my other issue that was backported to both versions. Thank you for pointing out, edited.

[bedevere-bot]

GH-108652 is a backport of this pull request to the 3.10 branch.