Equivalent of NotImplementedError for fields in Python

Question

In Python 2.x when you want to mark a method as abstract, you can define it like so:

class Base:
    def foo(self):
        raise NotImplementedError(\"Subcl         


        

Answer 1

A better way to do this is using Abstract Base Classes:

import abc

class Foo(abc.ABC):

    @property
    @abc.abstractmethod
    def demo_attribute(self):
        raise NotImplementedError

    @abc.abstractmethod
    def demo_method(self):
        raise NotImplementedError

class BadBar(Foo):
    pass

class GoodBar(Foo):

    demo_attribute = 'yes'

    def demo_method(self):
        return self.demo_attribute

bad_bar = BadBar()
# TypeError: Can't instantiate abstract class BadBar \
# with abstract methods demo_attribute, demo_method

good_bar = GoodBar()
# OK

Note that you should still have raise NotImplementedError instead of something like pass, because there is nothing preventing the inheriting class from calling super().demo_method(), and if the abstract demo_method is just pass, this will fail silently.