code: def land(self, velocity=VELOCITY):
          
          if self._is_flying:
              self.down(self._thread.get_height(), velocity)
  
              self._thread.stop()
              self._thread = None
  
              self._cf.commander.send_stop_setpoint()
              self._is_flying = False
docs: Go straight down and turn off the motors.
  
          Do not call this function if you use the with keyword. Landing is
          done automatically when the context goes out of scope.
  
          :param velocity: The velocity (meters/second) when going down
          :return: