Subclasses of abstract class that do not inherit abc.ABC are considered abstract

[sshane]

Bug description

Related to #3098. Subclasses of an abstract parent class which do not inherit directly from abc.ABC are considered abstract, and only throw an error during run time when trying to instantiate it.

For example, take this code below. PyCharm correctly determines through a IDE warning that Sub is not abstract, and says Class Sub must implement all abstract methods, but pylint will ignore this class, as its ancestors inherit from abc.ABC. Do we not get the best of both worlds (allowing abstract subclasses of abstract parent classes, ie. AbsSub, while catching non-abstract subclasses of abstract parent classes, ie. Sub) if we check the bases, instead of the ancestors where that PR to fix the issue above does it?

import abc
import unittest

class Abstract(unittest.TestCase, abc.ABC):  # abstract, should have no warning
  @abc.abstractmethod
  def test_something(self):
    pass

  @abc.abstractmethod
  def test_another_thing(self):
    pass


class AbsSub(Abstract, abc.ABC):  # abstract, should have no warning
  def test_another_thing(self):
    return 1


class Sub(AbsSub):  # not abstract since it doesn't inherit from abc.ABC directly, should warn
  pass

Configuration

No response

Command used

pylint test.py

Pylint output

`NOTHING`

Expected behavior

test.py:19:0: W0223: Method 'test_something' is abstract in class 'Abstract' but is not overridden (abstract-method) (referring to Sub)

Pylint version

2.5.0 (but same behavior on 2.15.8)

OS / Environment

Ubuntu 20.04

Additional dependencies

No response

[DanielNoord]

Seems like the check is indeed incorrect!

[sshane]

If not one wants to/has time to open a PR, I can try to tackle it.

[DanielNoord]

That would definitely be appreciated!

[DetachHead]

as a workaround, you can define your own version of ABC instead:

from abc import abstractmethod
import unittest

class ABC(metaclass=ABCMeta):
    pass

class Abstract(unittest.TestCase, ABC):
  @abstractmethod
  def test_something(self):
    pass

  @abstractmethod
  def test_another_thing(self):
    pass


class AbsSub(Abstract, abc.ABC):
  def test_another_thing(self):
    return 1


class Sub(AbsSub):  # error: Method 'test_something' is abstract in class 'Abstract' but is not overridden in child class 'Sub'
  pass