code: def on(self, left_speed, right_speed):
          
          (left_speed_native_units, right_speed_native_units) = self._unpack_speeds_to_native_units(left_speed, right_speed)
  
          # Set all parameters
          self.left_motor.speed_sp = int(round(left_speed_native_units))
          self.right_motor.speed_sp = int(round(right_speed_native_units))
  
          # This debug involves disk I/O to pull speed_sp so only uncomment
          # if you need to troubleshoot in more detail.
          # log.debug("%s: on at left-speed %s, right-speed %s" %
          #     (self, self.left_motor.speed_sp, self.right_motor.speed_sp))
  
          # Start the motors
          self.left_motor.run_forever()
          self.right_motor.run_forever()
docs: Start rotating the motors according to ``left_speed`` and ``right_speed`` forever.
          Speeds can be percentages or any SpeedValue implementation.