被 paho-mqtt 中的约定混淆，将内置方法分配为我创建的 function（但没有参数）

Question

I am trying to get my head around the paho-MQTT library. I am struggling to understand what is clearly a convention in the coding of the library, but which doesn't make sense to me. I am happy to look this up, if someone can give me the topic I should be looking for.

A lot of paho-mqtt tutorials and PAHO Foundation page ( https://www.eclipse.org/paho/index.php?page=clients/python/docs/index.php#constructor-reinitialise ) talk about handling the on_message , and on_connect methods (or at least what i think are methods) for the Client object. The tutorials all provide a standard way to engage with these methods, but in a way that I can't understand. It goes as follows:

Define a function that takes a set number of arguments. Something like:

def on_message(client, userdata, message):  
    print(message.payload, 'on', message.topic)

The process then is to create an 'mqtt Client' object and connect it to the broker. After that, to see the messages that client is subscribed to, i do the following:

client.on_message = on_message

This is this part that I don't understand. I understand this as meaning I am assigning a method the value of a function (but without calling its arguments or indicating its a function). I would have thought that client.on_message would have returned a 3 tupple, that I would have accessed via the function above, as follows:

on_message(client.on_message)

When I call type(client.on_message) I get NoneType back, indicating that mqtt.Client.on_message doesn't return anything. This explains why i can't call my function on the method.

Perhaps this is just a syntax issue, but could someone explain the convention here (or tell me what i should be looking up). Is Client.on_message a method of the class mqtt.Client? and how am I assigning it the value of a function I defined without providing any arguments, despite specifying arguments when I defined them? Further, how am I assigning the function without indicating the parenthesis ( on_message() )?

Below is a full bit of working code for paho-mqtt:

#import the library
import paho.mqtt.client as mqtt 

#Write the function to get the payload content (i.e. the text) from the message object
def on_message(client, userdata, message):
    print('Recieved message', str(message.payload))

#create the mqtt object and connect to the broker
MQTT_BROKER = [broker-IP]
client = mqtt.Client('Client1')
client.connect(MQTT_BROKER)

#subscribe to the topic
client.subscribe('TEST_TOPIC')

#Somehow invoke the function defined above on the mqtt on.message method - i.e. what i don't understand
client.on_message = on_message

#Do this continually so i can keep looking for messages published on this topic
client.loop_forever()

The problem I am having is that i still can't see messages in the console, on the topic to which I am subscribed. I know these are being published elsewhere (on another client) because I can see them on the broker when running mosquitto_sub -t 'TEST_TOPIC' .

At the moment I am just trying to understand the convention so that I can troubleshoot.

Answer 1

You will never explicitly call any of the callback functions. The client will call them from it's event loop at the appropriate time.

client.on_message = on_message is how yo tell the client which function to call (in the future) when a message arrives. Passing a function name with no arguments passes a handle to the function it's self.

Similar with the on_connect function, this is a function that the client will call once it has completed connecting to the broker.

I have re-arranged your code to ensure things are done in the right order using the on_connect callback to subscribe to the topic once it has finished connecting.

#import the library
import paho.mqtt.client as mqtt 

#Write the function to get the payload content (i.e. the text) from the message object
def on_message(client, userdata, message):
    print('Recieved message', str(message.payload))

def on_connect(client, userdata, flags, rc):
    #subscribe to the topic
    client.subscribe('TEST_TOPIC')

#create the mqtt object and connect to the broker
MQTT_BROKER = [broker-IP]
client = mqtt.Client('Client1')
client.on_connect = on_connect
client.on_message = on_message
client.connect(MQTT_BROKER)

#Do this continually so i can keep looking for messages published on this topic
client.loop_forever()