'Returns a final css class concatenated string.'

def get_final_css(self):

    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
