code: def on_message(self, sender, content):
          
          try:
              message = json.loads(content)
  
          except (ValueError, TypeError) as ex:
              logging.error("Not a valid JSON string: %s", ex)
              return
  
          try:
              # Check the replied message
              reply_uid = message['reply-to']
              reply_level = message['reply-level']
  
          except KeyError:
              # Got a new message
              logging.info("Got message %s from %s", message['content'], sender)
  
              # Notify listeners
              self.__pool.enqueue(self._notify_listeners, sender, message)
  
          else:
              # Got a reply
              try:
                  level, callback = self.__callbacks[reply_uid]
  
              except KeyError:
                  # Nobody to callback...
                  pass
  
              else:
                  if level == reply_level:
                      # Match
                      try:
                          callback(sender, message['payload'])
                      except Exception as ex:
                          logging.exception("Error notifying sender: %s", ex)
docs: Got a message from the client