How does mavutil.messages dictionaries in pymavlink works?

[xianglic]

Hi,

I am confused on how mavutil.messages work in pymavlink.
I have a orange cube ardupilot drone. And right after connection, I print out the field in mavutil.messages, it shows me “MAV” , “HOME” field, which I assume supposed to be default telemetry stream. And then, after I used the following code to loop receiving the msgs, I got some other fields from mavutil.messages like “ALTITUDE”, "GLOBAL_LOCATION_INT.

async def _message_listener(self):
    logger.info("-- Starting message listener")
    try:
        while True:
            msg = await asyncio.to_thread(self.vehicle.recv_match, blocking=True)
            if msg:
                message_type = msg.get_type()
                logger.debug(f"Received message type: {message_type}")
    except asyncio.CancelledError:
        logger.info("-- Message listener stopped")
    except Exception as e:
        logger.error(f"-- Error in message listener: {e}")

However, i dont want to loop receiving the the msg all the time for saving my computation resource. And I find the quote from this post.

@EliotBR

The Pixhawk will send a small set of vital messages by default, but other messages you’re interested in need to be requested. QGC requests a large set of messages automatically, whereas if you’re connecting directly to the Pixhawk with Pymavlink you’ll need to request those messages yourself. You can request a message to be sent regularly using MAV_CMD_SET_MESSAGE_INTERVAL, which we have a Pymavlink example for here

it seems like using this MAV_CMD_SET_MESSAGE_INTERVAL can make the autopilot to automatically send the nondefault message stream without requesting it. I tried it in my code without using the loop receiving mechanism. The mavutil.messages still shows only “MAV” and “HOME” field after the MAV_CMD_SET_MESSAGE_INTERVAL.

Can anyone explains this? is the communication model for pymavlink using request/reply or push/pull kind? when to properly use mavutil.messages? how does using recv_match() method affect the field stored in mavutil.messages?

[tony-white]

Hi @xianglic -
Welcome to the Blue Robotics forums! The focus is mainly on marine robotics here, which often don’t have GPS or other common features of ArduCopter or ArduPilot - your enquiry may be better suited to the ardupilot forum!

[EliotBR (Eliot)]

Hi @xianglic, welcome to the forum

@xianglic

how does using recv_match() method affect the field stored in mavutil.messages?

This is the key question here - the recv_match function is what handles processing all the received messages, so the latest messages don’t get updated if you don’t do the receiving part.

Some previous comments/threads that may help with understanding:

• some discussion of recv_match, and pymavlink more generally
• some (synchronous) approaches for receiving data
• some more discussion of pymavlink, including an asyncio example
• discussion of using MAVLink2REST instead of pymavlink
  • if you use MAVLink2REST, or a compatible router like MAVLink Server, you can hand off the initial receiving and filtering to a more performant application/service, and then your Python code can just fetch only the messages you actually want (at the cost of not having the Pymavlink niceties provided for you) - this is likely a good approach if performance/efficiency are important, and you don’t have lots of different message types you need to handle