CODEWITHSUNDEEP
pythonmqttmosquittopaho
Martin Ventura
Martin Ventura

Reputation: 197

Paho subscriber on_message/on_connect not working

I've installed Mosquitto and used in the terminal without problem, also installed paho for c++, both mosquitto and paho work well. Then installed paho for python via pip. Now i'm trying to test it with this code but the on_message and on_connect functions never get called. I'm doing

mosquitto_pub -h localhost -t "test" -m "This is a message"

while the python script runs but it doesn't work. It never gets to the line that prints "on_message!"

import paho.mqtt.client as mqtt

def on_connect(self, mqtt_client, obj, flags, rc):
    mqtt_client.subscribe("test")

def on_message(self, mqtt_client, obj, msg):
    print "on_message()"


print "Initializing subscriber"
mqtt_client = mqtt.Client()
mqtt_client.on_connect = on_connect
mqtt_client.on_message = on_message
mqtt_client.connect("localhost",1883)
print "Listening"
while True: 
    mqtt_client.loop()

What am i doing wrong?

Upvotes: 5

Views: 11473

Answers (4)

Rahul Rajeev
Rahul Rajeev

Reputation: 1

I had a similar problem with the on_message function not working in Python Paho. After some troubleshooting, I discovered that I was subscribed to the wrong topic. Since I wasn’t subscribed to the correct topic, my on_message function wasn’t being triggered.

The on_message function works when you receive a message on a subscribed topic. Therefore, to receive messages, you need to ensure that you are subscribed to the correct topic.

This is a simple mistake, but it can happen. I hope sharing my experience helps others who might face the same issue.

Upvotes: 0

PeteC
PeteC

Reputation: 149

This is an old thread but I just hit a similar problem. I had:

def on_message (client, userdata,message):
      print("received message", str(message.payload.decode("utf-8")

client.loop_start()
client.subscribe(topic)
client.on_message=on_message
time.sleep(30)
client.loop_stop()

If I changed the time from 30 to 300 then on_message was never executed. Go figure?

Upvotes: 0

Aleksandar
Aleksandar

Reputation: 4154

The on_message callback gets called only if You are subscribed to a topic. To do that, You need to call the subscribe method. For example:

print "Initializing subscriber"
mqtt_client = mqtt.Client()
mqtt_client.on_connect = on_connect
mqtt_client.on_message = on_message
mqtt_client.connect("localhost", 1883) # "localhost" or IP or 
mqtt_client.subscribe("test") # topic name, same as the "-t" argument in mosquitto_pub
print "Listening"
mqtt_client.loop_forever()

As for the on_connect callback, what caused the issue for me was that I called the connack_string method as it was Client's method, but it is not. Looking at the source code, we see that it is defined directly in the module, not inside a class:

def connack_string(connack_code):

So, I replaced this:

def connect_callback(client, userdata, flags, rc):
    print("Connection attempt returned: " + client.connack_string(rc)) # client !!

with this:

def connect_callback(client, userdata, flags, rc):
    print("Connection attempt returned: " + mqtt.connack_string(rc)) # mqtt !!

What You can also try is to omit the -h localhost part from the mosquitto_pub command, like this:

mosquitto_pub -t "test" -m "This is a message"

Also, watch for the number of parameters that the on_connect callback receives (4). This is advice to everybody else reading this post. :)

Upvotes: 4

hardillb
hardillb

Reputation: 59816

The following is working fine for me.

I have removed the self from the callbacks and swapped the loop for mqtt_client.loop_forever() 

import paho.mqtt.client as mqtt

def on_connect(mqtt_client, obj, flags, rc):
    mqtt_client.subscribe("test")

def on_message(mqtt_client, obj, msg):
    print "on_message()"


print "Initializing subscriber"
mqtt_client = mqtt.Client()
mqtt_client.on_connect = on_connect
mqtt_client.on_message = on_message
mqtt_client.connect("localhost",1883)
print "Listening"
mqtt_client.loop_forever()

Upvotes: 1

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