The 'Abstract Base classes" or abc.ABC is a helper class

https://docs.python.org/3/library/abc.html

Here's a snippet of why they exist:

The collections module has some concrete classes that derive from ABCs; these can, of course, be further derived. In addition, the collections.abc submodule has some ABCs that can be used to test whether a class or instance provides a particular interface, for example, if it is hashable or if it is a mapping.

A good example here: https://pymotw.com/2/abc/ | https://pymotw.com/3/abc/

From pymotw:

Forgetting to set the metaclass properly means the concrete implementations do not have their APIs enforced. To make it easier to set up the abstract class properly, a base class is provided that sets the metaclass automatically.

abc_abc_base.py
import abc


class PluginBase(abc.ABC):

    @abc.abstractmethod
    def load(self, input):
        """Retrieve data from the input source
        and return an object.
        """

    @abc.abstractmethod
    def save(self, output, data):
        """Save the data object to the output."""


class SubclassImplementation(PluginBase):

    def load(self, input):
        return input.read()

    def save(self, output, data):
        return output.write(data)


if __name__ == '__main__':
    print('Subclass:', issubclass(SubclassImplementation,
                                  PluginBase))
    print('Instance:', isinstance(SubclassImplementation(),
                                  PluginBase))