Returns a final css class concatenated string.

def get_final_css(self):
        """Returns a final css class concatenated string."""
        default = " ".join(self.get_default_classes())
        defined = self.attrs.get('class', '')
        additional = " ".join(getattr(self, "classes", []))
        non_empty = [test for test in (defined, default, additional) if test]
        final_classes = " ".join(non_empty).strip()
        return final_classes