Python Flask server with MQTT subscription

You need to pass the details of the broker you want to connect to in the connect() function

e.g.

...client.on_message = on_messageclient.connect('broker.example.com')client.subscribe(topic)...

EDIT: you also need to start the network loop. Given this is flask app you will want to start the loop in the background so use the loop_start function.

...client.connect('broker.example.com')client.loop_start()...

You should also move the subscribe and publish calls to the on_connect callback as they need to wait for the connection to have been established before being run.

EDIT2: adding full working code:

from flask import Flaskimport paho.mqtt.client as mqttapp = Flask(__name__)topic = 'foo'topic2 = 'bar'port = 5000def on_connect(client, userdata, rc):    client.subscribe(topic)    client.publish(topic2, "STARTING SERVER")    client.publish(topic2, "CONNECTED")def on_message(client, userdata, msg):    client.publish(topic2, "MESSAGE")@app.route('/')def hello_world():    return 'Hello World! I am running on port ' + str(port)if __name__ == '__main__':    client = mqtt.Client()    #client.username_pw_set(username, password)    client.on_connect = on_connect    client.on_message = on_message    client.connect('localhost')    client.loop_start()    app.run(host='0.0.0.0', port=port)

You can use also Flask-MQTT:

https://flask-mqtt.readthedocs.io/en/latest/usage.html

There is even functional example how to implement MQTT-SocketIO bridge, which is just what I was looking for. This worked out-of-the-box:

https://flask-mqtt.readthedocs.io/en/latest/usage.html#interact-with-socketio