When should one inherit from ABC?

Question

I always thought one should inherit from abc.ABC when one does not want the class to be instantiated. But I've just realized that if a class has an @abstractmethod then one can also not instanciate it.

Is there any other reason to inherit from ABC?

Answer 1

Unless you use abc.ABCMeta as the metaclass for your class (either explicitly or by inheriting from abc.ABC), using abstractmethod doesn't really do anything.

>>> from abc import abstractmethod, ABC
>>> class Foo:
...   @abstractmethod
...   def bar(self):
...     pass
...
>>> f = Foo()
>>>

Likewise, using ABCMeta doesn't mean much unless you mark at least one method as abstract:

>>> class Bar(ABC):
...     pass
...
>>> b = Bar()
>>>

It's the combination of the two that allows a class to be (nominally) uninstantiable:

>>> class Baz(ABC):
...   @abstractmethod
...   def m(self):
...     pass
...
>>> b = Baz()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Baz with abstract methods m
>>>

(Even then, note that all @abstractmethod does is add the decorated method to a set which the metaclass machinery consults when trying to instantiate the class. It is trivial to defeat that machinery:

>>> Baz.__abstractmethods__
frozenset({'m'})
>>> Baz.__abstractmethods__ = set()
>>> b = Baz()
>>>

)

---

Note that ABC itself is a trivial class that uses ABCMeta as its metaclass, which makes any of its descendants use it as well.

# Docstring omitted; see
# https://github.com/python/cpython/blob/3.7/Lib/abc.py#L166
# for the original
class ABC(metaclass=ABCMeta):
    __slots__ = ()