Question

I have a “partial” class that requires some mixin for its functionality (I want to do it with inheritance for performance and simplicity reasons). Can I declare that my class is going to need new methods?

Apparently the following guess does not work (“Can’t instantiate abstract class”):

from abc import abstractmethod, ABCMeta

class A(metaclass=ABCMeta):
    @abstractmethod
    def a(self):
        pass

class B:
    def a(self):
        return 12

class C(A, B):
    pass

c = C()

Here A tries to declare that its other methods need a() to work.
(Python 3)

Any suggestions to the declare that?

Answer 1

You need to put B in front of A in the inheritance order for that to work:

>>> from abc import abstractmethod, ABCMeta

>>> class A:
...     __metaclass__=ABCMeta
...     @abstractmethod
...     def a(self):
...         pass

>>> class B:
...     def a(self):
...         return 12

>>> class C(B,A):
...     pass

>>> c = C()
>>> c
<__main__.C object at 0x10e9c40d0>


>>> class D:
...     pass

>>> class E(D, A):
...     pass

>>> e = E()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: Can't instantiate abstract class E with abstract methods a