If a subclass has abstract methods, it fails to instantiate... unless it's a metaclass, and then it succeeds.

>>> import abc
>>> class A(metaclass=abc.ABCMeta):
...     @abc.abstractmethod
...     def foo(self): pass
... 
>>> class B(A): pass
... 
>>> B()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class B with abstract methods foo
>>> class C(A, type): pass
... 
>>> class c(metaclass=C): pass
... 
>>> C('', (), {})
<class '__main__.'>
>>>