code: def add_payload(self, key, val, append=False):
          
          if append:
              self._payload.setdefault(key, []).append(val)
          else:
              self._payload[key] = val
docs: Add a key value pair to payload for this request.
  
          Args:
              key (str): The payload key.
              val (str): The payload value.
              append (bool, default:False): Indicate whether the value should be appended or
                  overwritten.