How to find extension classes. Python

extensions.py

import sys
import inspect

class A:
    pass

class B(A):
    pass


def find_extentions(interface, module=__name__):
    module = sys.modules[module]
    clsmembers = inspect.getmembers(module, inspect.isclass)
    extentions = []

    for clsmember_name, clsmember in clsmembers:
        if clsmember != interface and issubclass(clsmember, interface):
            extentions.append(clsmember)

    return extentions


print(find_extentions(A))