code: def attach(self, payload):
          
          if self._payload is None:
              self._payload = [payload]
          else:
              self._payload.append(payload)
docs: Add the given payload to the current payload.
  
          The current payload will always be a list of objects after this method
          is called.  If you want to set the payload to a scalar object, use
          set_payload() instead.