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

[blhsing]

Bug report

Bug description:

As reported in Discourse, 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'

We should make the unittest loaders avoid loading such test cases and methods from them.

CPython versions tested on:

3.12, CPython main branch

Operating systems tested on:

Linux, Windows

Linked PRs

• gh-120665: make unittest loaders avoid loading test cases that are abstract base classes #120666
• [3.13] gh-120665: make unittest loaders avoid loading test cases that are abstract base classes (GH-120666) #151599
• [3.14] gh-120665: make unittest loaders avoid loading test cases that are abstract base classes (GH-120666) #151600
• [3.15] gh-120665: make unittest loaders avoid loading test cases that are abstract base classes (GH-120666) #151601

[serhiy-storchaka]

See also previous discussion in #58739.

[JonathonReinhart]

Until this issue is resolved, I hope this little snippet helps someone:

import inspect
import unittest

# ...

class MyTestLoader(unittest.TestLoader):
    def loadTestsFromTestCase(self, testCaseClass):
        if inspect.isabstract(testCaseClass):
            return self.suiteClass()
        return super().loadTestsFromTestCase(testCaseClass)


if __name__ == "__main__":
    unittest.main(verbosity=2, testLoader=MyTestLoader())