docs: def get_attribute(self, attribute_name, mapped_class=None, key=None):
          """
          Returns the specified attribute from the map of all mapped attributes
          for the given mapped class and attribute key. See
          :method:`get_attribute_map` for details.
          """
          attr_map = self.__get_attribute_map(mapped_class, key, 0)
          try:
              return attr_map[attribute_name]
          except KeyError:
              raise AttributeError(attribute_name)