code: def callback(self, msg):
          
          message_type, channel, payload = msg
  
          if message_type != 'message':
              return
  
          try:
              payload = self._decode(payload)
          except ValueError:
              # unable to decode so we cannot handle the message
              return
  
          if channel == self.group_pubsub:
              action_f = self.action
          elif channel in self._listening_to:
              action_f = self.job_action
          else:
              raise ValueError("Callback triggered unexpectedly by %s" % channel)
  
          for verb, args in payload.items():
              action_f(verb, args)
docs: Accept a message that was published, process and forward
  
          Parameters
          ----------
          msg : tuple, (str, str, str)
              The message sent over the line. The `tuple` is of the form:
              (message_type, channel, payload).
  
          Notes
          -----
          This method only handles messages where `message_type` is "message".
  
          Raises
          ------
          ValueError
              If the channel is not known.