code: def _get_or_add(self, prop_name):
          
          get_or_add_method_name = 'get_or_add_%s' % prop_name
          get_or_add_method = getattr(self, get_or_add_method_name)
          element = get_or_add_method()
          return element
docs: Return element returned by 'get_or_add_' method for *prop_name*.