3.13.0b3 regression: inspect.signature + unittest.mock

[neutrinoceros]

Bug report

Bug description:

MRE:

import inspect
import functools
from unittest import TestCase
import unittest.mock as mk

class TestIsinstance(TestCase):
    def test_mk(self):
        def foo(): pass
        with mk.patch.object(functools, "partial", autospec=True):
            inspect.signature(foo)

fails with

E
======================================================================
ERROR: test_mk (test.TestIsinstance.test_mk)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/private/tmp/reprod/test.py", line 10, in test_mk
    inspect.signature(foo)
    ~~~~~~~~~~~~~~~~~^^^^^
  File "/Users/clm/.pyenv/versions/3.13.0b3/lib/python3.13/inspect.py", line 3333, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                   globals=globals, locals=locals, eval_str=eval_str)
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/clm/.pyenv/versions/3.13.0b3/lib/python3.13/inspect.py", line 3059, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
                                    follow_wrapper_chains=follow_wrapped,
                                    globals=globals, locals=locals, eval_str=eval_str)
  File "/Users/clm/.pyenv/versions/3.13.0b3/lib/python3.13/inspect.py", line 2559, in _signature_from_callable
    if isinstance(obj, functools.partial):
       ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

----------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (errors=1)

reproduces locally and in CI: example logs

this test case is adapted from a real life application in astropy's test suite, which has been stable since at least Python 3.8

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, 3.12, 3.13

Operating systems tested on:

Linux, macOS, Windows

[picnixz]

Could it be because of #121092? (especially: https://github.com/python/cpython/pull/121092/files#diff-b89cd06e4637abacb73f2500301ff979a67ad7ecbfa7cf151c7243715898d7eaL2570-L2573)

[neutrinoceros]

Thank you ! It's definitely related: the test pass if I revert just the bit you pointed to.

[picnixz]

cc @serhiy-storchaka

[serhiy-storchaka]

It is not safe to patch functools.partial which is used in the inspect module and then use the inspect module.

Add the following code in your example and you will see that it was broken before #121092.

            inspect.signature(self.test_mk)
            def bar(x: int) -> None: pass
            inspect.get_annotations(bar, eval_str=True)

[neutrinoceros]

Thanks for pointing this out !
I was able to resolve this on our side by avoiding mocking altogether at astropy/astropy#16659
Feel free to close this issue.