code: def condition_yaw(heading, relative=False):
      
      if relative:
          is_relative = 1 #yaw relative to direction of travel
      else:
          is_relative = 0 #yaw is an absolute angle
      # create the CONDITION_YAW command using command_long_encode()
      msg = vehicle.message_factory.command_long_encode(
          0, 0,    # target system, target component
          mavutil.mavlink.MAV_CMD_CONDITION_YAW, #command
          0, #confirmation
          heading,    # param 1, yaw in degrees
          0,          # param 2, yaw speed deg/s
          1,          # param 3, direction -1 ccw, 1 cw
          is_relative, # param 4, relative offset 1, absolute angle 0
          0, 0, 0)    # param 5 ~ 7 not used
      # send command to vehicle
      vehicle.send_mavlink(msg)
docs: Send MAV_CMD_CONDITION_YAW message to point vehicle at a specified heading (in degrees).
  
      This method sets an absolute heading by default, but you can set the `relative` parameter
      to `True` to set yaw relative to the current yaw heading.
  
      By default the yaw of the vehicle will follow the direction of travel. After setting 
      the yaw using this function there is no way to return to the default yaw "follow direction 
      of travel" behaviour (https://github.com/diydrones/ardupilot/issues/2427)
  
      For more information see: 
      http://copter.ardupilot.com/wiki/common-mavlink-mission-command-messages-mav_cmd/#mav_cmd_condition_yaw