python source code of subscribe_with

"""
Book: Hands-On MQTT Programming with Python
Author: Gaston C. Hillar - Twitter.com/gastonhillar
Publisher: Packt Publishing Ltd. - http://www.packtpub.com
"""
from config import *
import paho.mqtt.client as mqtt


def on_connect(client, userdata, flags, rc):
    print("Result from connect: {}".format(
        mqtt.connack_string(rc)))
    # Subscribe to the vehicles/vehiclepi01/tests topic filter
    client.subscribe("vehicles/vehiclepi01/tests", qos=2)


def on_subscribe(client, userdata, mid, granted_qos):
    print("I've subscribed with QoS: {}".format(
        granted_qos[0]))


def on_message(client, userdata, msg):
    print("Message received. Topic: {}. Payload: {}".format(
        msg.topic, 
        str(msg.payload)))


if __name__ == "__main__":
    client = mqtt.Client(protocol=mqtt.MQTTv311)
    client.on_connect = on_connect
    client.on_subscribe = on_subscribe
    client.on_message = on_message
    client.tls_set(ca_certs = ca_certificate,
        certfile=client_certificate,
        keyfile=client_key)
    client.connect(host=mqtt_server_host,
        port=mqtt_server_port,
        keepalive=mqtt_keepalive)
    client.loop_forever()