Adds @items to the end of the list
            -> #int length of list after operation

def extend(self, items):
        """ Adds @items to the end of the list
            -> #int length of list after operation
        """
        if items:
            if self.serialized:
                items = list(map(self._dumps, items))
            self._client.rpush(self.key_prefix, *items)