v8.2.0 raises `Can't instantiate abstract class` when a test class implements an abstract class

[MartinBraquet]

Before, we could have a test class inheriting from an ABC (and unittest.TestCase), now it errors because pytest is trying to instantiate the ABC itself. Running pytest should only instantiate the child test class (which indeed implements all the abstract methods of its ABC parent).
This issue does not appear in v8.1.2 and was likely introduced in #12089.

Minimal example:

from abc import ABC, abstractmethod
from unittest import TestCase


class Base(TestCase, ABC):
    @abstractmethod
    def foo(self): ...


class Tests(Base):

    def foo(self): ...

    def test(self): ...

Output:

(test) tests 🔥pytest test.py    
============================================================================= test session starts ==============================================================================
platform darwin -- Python 3.9.19, pytest-8.2.0, pluggy-1.5.0
rootdir: ...
configfile: pytest.ini
plugins: env-1.1.3
collected 1 item / 1 error                                                                                                                                                     

==================================================================================== ERRORS ====================================================================================
________________________________________________________________ ERROR collecting test.py ________________________________________________________________
/Users/martin/miniconda3/envs/test/lib/python3.9/site-packages/_pytest/runner.py:341: in from_call
    result: Optional[TResult] = func()
/Users/martin/miniconda3/envs/test/lib/python3.9/site-packages/_pytest/runner.py:389: in collect
    return list(collector.collect())
/Users/martin/miniconda3/envs/test/lib/python3.9/site-packages/_pytest/unittest.py:90: in collect
    self.session._fixturemanager.parsefactories(self.newinstance(), self.nodeid)
/Users/martin/miniconda3/envs/test/lib/python3.9/site-packages/_pytest/unittest.py:75: in newinstance
    return self.obj("runTest")
E   TypeError: Can't instantiate abstract class Base with abstract method foo
=========================================================================== short test summary info ============================================================================
ERROR test.py::Base - TypeError: Can't instantiate abstract class Base with abstract method foo
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
=============================================================================== 1 error in 0.04s ===============================================================================

pip list:

Package        Version
-------------- -------
exceptiongroup 1.2.1
iniconfig      2.0.0
packaging      24.0
pip            23.3.1
pluggy         1.5.0
pytest         8.2.0
pytest-env     1.1.3
setuptools     68.2.2
tomli          2.0.1
wheel          0.41.2

Thanks for your work in general, it's a great package.

[The-Compiler]

Your suspicion is correct, a bisect indeed points to 1a5e0eb.

[bluetech]

Thanks for the report. I agree that what you're doing is very reasonable and should work.

Until we have some fix for this, if you want to remove the pin, a workaround would be to drop this snippet to a conftest.py file in the directory where the abc is defined:

import inspect

def pytest_pycollect_makeitem(collector, name, obj):
    if inspect.isabstract(obj):
        return []

[bluetech]

Since I don't currently have time to attempt the change hinted at the final sentence in #12089, I suggest we just don't collect abstract classes for now. It probably makes some sense anyway -- the problem also exists for non-unittest test classes, so might as well fix it there as well.

[bluetech]

This will be fixed in pytest 8.3.0 (this fix is a bit too scary to backport to 8.2.x).