def _walk_mro(cls):
        """Walk the cls.mro() for parent classes that are also singletons

        For use in instance()
        """

        for subclass in cls.mro():
            if issubclass(cls, subclass) and \
                    issubclass(subclass, SingletonConfigurable) and \
                    subclass != SingletonConfigurable:
                yield subclass
Walk the cls.mro() for parent classes that are also singletons

        For use in instance()