code: def update(self, sensor, reading):
          
          parents = list(self._child_to_parents[sensor])
          for parent in parents:
              self.recalculate(parent, (sensor,))
docs: Update callback used by sensors to notify obervers of changes.
  
          Parameters
          ----------
          sensor : :class:`katcp.Sensor` object
              The sensor whose value has changed.
          reading : (timestamp, status, value) tuple
              Sensor reading as would be returned by sensor.read()