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

def append(self, items):
        """ 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


        """
        resp = self.client.add_to_item_list(items, self.url())
        self.refresh()
        return resp