code: def append(self, items):
          
          resp = self.client.add_to_item_list(items, self.url())
          self.refresh()
          return resp
docs: Add some items to this ItemList and save the changes to the server
  
          :param items: the items to add, either as a List of Item objects, an
              ItemList, a List of item URLs as Strings, a single item URL as a
              String, or a single Item object
  
          :rtype: String
          :returns: the server success message
  
          :raises: APIError if the API request is not successful