https://medium.com/jungletronics/mosquitto-meets-raspberry-pi-part-1-4b321a77802c

mqtt_sub_temp.py

# # Python      mqtt_sub_temp.py
# This program access Mysql inside PhpMyAdmin Application on my Raspberry PI
# It subscribe (as admin) to a Topic 'temp' the value 25 ° C
# Please create a User named user1, password 321 in your Pi
import paho.mqtt.client as mqtt
import requests
import sys
import mysql.connector
from datetime import date, datetime, timedelta
import time

# Mosquitto MQTT
username 	= "admin"
password 	= "123"
hostname 	= "localhost"
port 		= 1883

# Banco de dados MySql
user_mysql 	= "pi"
pwd_mysql 	= "root"
db_mysql	= "mydb"

# Callback event functions
# on_connect = function called when the connection between the client and the MQTT broker occurs
# on_message = function called when a message from a subscribed topic is received
# on_publish = function called when a message is published 
# on_subscribe = function called when a topic is subscribed to by the MQTT client
# on_log = function for debugging Paho
def on_connect(client, userdata, flags, rc):
    print("Method-> on_connect: " + str(rc))

def on_message(client, obj, msg):
    # Print the message content
    print(msg.topic + " " + str(msg.qos) + " " + str(msg.payload) )
	
    # Connection to Mysql database
    # All configuration parameters of the Mysql Connector can be found
    # Here: https://dev.mysql.com/doc/connector-python/en/connector-python-connectargs.html
	
    cnx = mysql.connector.connect(user=user_mysql, password=pwd_mysql, database=db_mysql)
    cursor = cnx.cursor()
    query = ("INSERT INTO sensors "
            "(name, value, timestamp) "
            "VALUES (%s, %s, %s)")
    # msg.topico = stores the topic
    # msg.payload = stores the message content
    # time.time = timestamp (UTC).
    dados_recebidos = (msg.topic, msg.payload, str(int(time.time())))
	
    # Load and run the query.
    cursor.execute(query, dados_recebidos)
    cnx.commit()
	
    # Terminates the connection to the database
    cursor.close()
    cnx.close()

def on_publish(client, obj, mid):
    print("Method-> on_publish: " + str(mid))

def on_subscribe(client, obj, mid, granted_qos):
    print("Method-> Subscribe: " + str(mid) + " " + str(granted_qos))

def on_log(client, obj, level, string):
    print(string)

#------------------------------------------------
mqttc = mqtt.Client()

# Sign the callback functions
mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_publish = on_publish
mqttc.on_subscribe = on_subscribe

# Uncomment to enable debug messages
#mqttc.on_log = on_log

# Connects to the MQTT Mosquitto 
# Enter the username and password 
mqttc.username_pw_set(username, password)

# Load the TLS certificate.
#mqttc.tls_set("/etc/mosquitto/certs/ca.crt")

# Enter the IP or DNS address of the Mosquitto server 
# Use 1883 for WITHOUT TLS connection, and 8883 for secure TLS connection
mqttc.connect(hostname, port)


# Sign the topic, with QoS level 0 (can be 0, 1 or 2
mqttc.subscribe("temp", 0)

# To post to a topic, use the following function
mqttc.publish("temp", "25")

# It remains in a loop even if there are errors
while True:
    try:
        rc = 0
        while rc == 0:
            rc = mqttc.loop()
        print("Mosquitto is running!: " + str(rc))
    except OtherExceptions:
        print("Exceptions Error Occurred!")