Need help understanding events in mqtt_as.py

[tferrin]

I'm a newbie when it comes to using the event interface in mqtt_as.py. I've tried the range_ex.py example in the mqtt_as distribution and I've cookie-cuttered code from that example into my own code and both work great. But there's a code construct that I just don't understand:

from mqtt_as import MQTTClient
client = MQTTClient(config)

async def up(client):
    while True:
        await client.up.wait()
        client.up.clear()
        wifi_led(True)
        print('We are connected to broker.')
        await client.subscribe(sensorTopic, 1)

How is this legal Python? MQTTClient doesn't have an "up" method (or "down" method either), so how can the line "await client.up.wait()" not raise an error? I know for this code to work it requires "config["queue_len"] = 4" but I don't understand how this this apparently changes the semantics of the statement "client.up.wait()". Can someone please point me to documentation that explains what's going on. Thanks!

[andrewleech]

up and down here are asyncio.Event() objects, see https://github.com/peterhinch/micropython-mqtt/blob/f9290c68b0459e2841e0565cb6b45bc636f698bd/mqtt_as/mqtt_as.py#L154

So the code is (async) waiting for these events to be set elsewhere in the mqtt library.

[tferrin]

Well okay, then clearly I need to do a deeper dive into asyncio.Event() functionality. I guess I had superficially read the documentation where it says "asyncio.Event creates a new event which can be used to synchronize tasks" but didn't fully appreciate what this means. Will look for some tutorials. Thanks!

[peterhinch]

You might like to try this tutorial.

Re mqtt_as the module offers two alternative interfaces, traditional callbacks or Event-based. The choice is yours; some demos use callbacks, some use Events. The choice determines what happens when an asynchronous event occurs, such as a message arriving. The callback concept is probably familiar to you. In the Event case the user runs a task which waits on the Event and then handles the message. In the Event case mqtt_as creates a queue: this avoids message loss in the case where messages might arrive in a burst whose rate briefly exceeds the speed at which the user task can process them.

In my opinion using tasks and Event instances is more in the spirit of asyncio than callbacks.

[tferrin]

OK. I got it now. The asyncio tutorial helped, as did a careful reading of section 3.5 Event based interface of the mqtt_as documentation. I originally got confused because when I looked at mqtt_as.py I didn't find either an "up" or "down" method, and it was, like, "how can this possibly work?"

And I'm all for the Event-based approach to mqtt_as. It was my desire to move away from callbacks that got me started down this path.

Thanks for the answers!