def _class_tags(cls):  # pylint: disable=no-self-argument
        """
        Collect the tags from all base classes.
        """
        class_tags = set()

        for base in cls.mro()[1:]:  # pylint: disable=no-member
            class_tags.update(getattr(base, '_class_tags', set()))

        return class_tags
Collect the tags from all base classes.