code: def trigger_callback(self, sid, namespace, id, data):
          
          callback = None
          try:
              callback = self.callbacks[sid][namespace][id]
          except KeyError:
              # if we get an unknown callback we just ignore it
              self._get_logger().warning('Unknown callback received, ignoring.')
          else:
              del self.callbacks[sid][namespace][id]
          if callback is not None:
              callback(*data)
docs: Invoke an application callback.