How to inherit from ABC and another class

Question

I have a class

class A(ABC):
  @abstractmethod
  def m():
    pass

and a bunch of classes inheriting from it.

class M1(A):
  ...

class M2(A):
  ...

...

Some of these have a lot in common as they share a lot of functionality such that I would like to introduce a common base class which itself inherits from A , but it does not introduce any new @abstractmethod methods:

       A
       |
  +----+----+
  |    |    |
  M1   M2   B
            |
         +--+--+
         |     |
         K1   K2

What should the definition for class B look like?

I'm on Python 3.8

EDIT: I am specifically asking as PyCharm tells me "B must inherit all abstract methods"

Answer 1

You can simply use

class B(A):
   pass

or any other code that is common to all children, but without m . This will still be abstract due to the absence of m .

---

An alternative is to use

class B(A, ABC):
   pass

In my opinion this is less good. This states that B is both an A and an ABC , while the former alternative states that B is an A , and it is an ABC insofar as much as A is an ABC itself.

Consider the case where you decide later on that A is not an ABC after all. For M1 and M2 , no change needs to be made. For B , you now need to remove its being an ABC as well. While making sure that A and B are synchronized in subclassing ABC is not a huge overhead in terms of code maintenance, I think it does indicate that the design is problematic, PyCharm's warning about the former alternative notwithstanding (I would suppress it as a false-positive).