Discussion on: Snake Eyes: Extension Methods

def extension(scope_cls):
    def _default(_obj, _name):
        raise AttributeError()

    # (1)
    # we access the base-class of our extension class to get the class we're extending.
    base_cls = scope_cls.__base__
    cls_name = scope_cls.__name__
    original_getattr = getattr(base_cls, '__getattr__', _default)

    def _getattr(obj, name):
        with suppress(AttributeError):
            return original_getattr(obj, name)

        # (2)
        # we check whether the requested attribute exists in our extension class.
        # As you can see, the changes are pretty simple and straight-forward.
        if not hasattr(scope_cls, name):
            raise AttributeError()

        # (3)
        #  we make the most important change -
        #  we check for the extension class in the scope,
        #  not the extension methods.
        #  This is the core of this change!
        if not _is_in_scope(scope_cls, cls_name):
            raise AttributeError()

        # (4)
        # we get the required attribute from out extension class.
        attribute = getattr(scope_cls, name)

        # if it is function, return as function
        if hasattr(attribute, '__call__'):
            return functools.partial(attribute, obj)

        # return as is
        return attribute

    cls.__getattr__ = _getattr

    return scope_cls

# no defined method here
class Context:
    def __init__(self):
        self.bag = dict()


@extension
class SpecificContext(Context):
    SPECIFIC_KEY = "SPECIFIC_KEY"

    def get_specific(self) -> Any:
        # bag will be taken from base class
        # SPECIFIC_KEY will be taken from extension class
        return self.bag[self.SPECIFIC_KEY]

    def set_specific(self, value: Any) -> None:
        self.bag[self.SPECIFIC_KEY] = value


def try_use_via_extension(context: SpecificContext) -> bool:
    # without @extension attribute this will fail
    # with exception 'no such attribute "set_specific" in Context class'
    context.set_specific("my-secret-here")

    key = context.get_specific()
    print("my secret is " + key)

    # return true because it does work :)
    return True


base_context = Context()  # of type Context
try_use_via_extension(base_context)  # but will be handled as SpecificContext