code: def future_check_sensor(self, name, update=None):
          
          exist = False
          yield self.until_data_synced()
          if name in self._sensors_index:
              exist = True
          else:
              if update or (update is None and self._update_on_lookup):
                  yield self.inspect_sensors(name)
                  exist = yield self.future_check_sensor(name, False)
  
          raise tornado.gen.Return(exist)
docs: Check if the sensor exists.
  
          Used internally by future_get_sensor. This method is aware of
          synchronisation in progress and if inspection of the server is allowed.
  
          Parameters
          ----------
          name : str
              Name of the sensor to verify.
          update : bool or None, optional
              If a katcp request to the server should be made to check if the
              sensor is on the server now.
  
          Notes
          -----
          Ensure that self.state.data_synced == True if yielding to
          future_check_sensor from a state-change callback, or a deadlock will
          occur.