Why does this abstract static method return None?

Question

The concrete class doesn't implement foo()

import abc

class Base(abc.ABC):

    @staticmethod
    @abc.abstractmethod
    def foo():
        ...

class Concrete(Base):
    pass

print(Concrete.foo())  # prints "None"

I'd expect this to fail with an error

Answer 1

It turns out that classes defined like:

def foo():
    ...

Will return None when called. If I want to communicate "hey you forgot to implement this" on a static method in a context where the caller never creates an object, I need to explicitly raise that error in the base class:

import abc
class Base:

    @staticmethod
    @abc.abstractmethod
    def foo():
        raise NotImplementedError("subclasses must implement this")

class Concrete(Base):
    pass

The expected error will show up if the caller attempts to create an object like below.

obj = Concrete()  # TypeError: Can't instantiate abstract class Concrete with abstract methods foo

However, if Concrete does define foo() it will be called instead.