gh-120665: make unittest loaders avoid loading test cases that are abstract base classes

[blhsing]

Make unittest loaders avoid loading test cases that are abstract base classes

Currently, loading a TestCase-derived subclass that is also an abstract base class like the following:

import unittest
from abc import ABC, abstractmethod

class FooBase(unittest.TestCase, ABC):
    @abstractmethod
    def test(self): ...

class Foo(FooBase):
    def test(self): pass

unittest.main()

would currently result in:

TypeError: Can't instantiate abstract class FooBase without an implementation for abstract method 'test'

This PR fixes the error by making the unittest loaders avoid loading such test cases and methods from them.

• Issue: unittest loaders should avoid loading test cases that are abstract base classes #120665

[clayg]

@nedbat I think your comments on https://discuss.python.org/t/unittest-ignore-test-cases-that-are-also-abc/55958/14 suggest an understanding of how this would be a great QoL improvement for test authors. Please help it get merged upstream.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[JonathonReinhart]

This seems like such a common-sense enhancement, I'm surprised it's being debated so heavily.

The argument that this could break existing code is invalid; existing code that hits this path would already raise an exception and never run.

And the intent of class FooBase(unittest.TestCase, ABC): is extremely clear IMO.

Please consider landing this change.

[miss-islington-app]

Thanks @blhsing for the PR, and @nedbat for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @blhsing for the PR, and @nedbat for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @blhsing for the PR, and @nedbat for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-app]

GH-151599 is a backport of this pull request to the 3.13 branch.

[bedevere-app]

GH-151600 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-151601 is a backport of this pull request to the 3.15 branch.