Example: receive messages (subscribe) with MQTT
This example describes how a client would receive messages from within a specific topic on the broker:
import paho.mqtt.client as mqtt
def on_connect(client, userdata, flags, rc): # The callback for when the client connects to the broker
print("Connected with result code {0}".format(str(rc))) # Print result of connection attempt
client.subscribe("digitest/test1") # Subscribe to the topic “digitest/test1”, receive any messages published on it
def on_message(client, userdata, msg): # The callback for when a PUBLISH message is received from the server.
print("Message received-> " + msg.topic + " " + str(msg.payload)) # Print a received msg
client = mqtt.Client("digi_mqtt_test") # Create instance of client with client ID “digi_mqtt_test”
client.on_connect = on_connect # Define callback function for successful connection
client.on_message = on_message # Define callback function for receipt of a message
# client.connect("m2m.eclipse.org", 1883, 60) # Connect to (broker, port, keepalive-time)
client.connect('127.0.0.1', 17300)
client.loop_forever() # Start networking daemon
Note You can easily copy and paste code from the online version of this guide. Use caution with the PDF version, as it may not maintain essential indentations.
The first line imports the library functions for MQTT.
The functions on_connect and on_message are callback functions which are automatically called by the client upon connection to the broker and upon receiving a message, respectively.
The on_connect function prints the result of the connection attempt, and performs the subscription. It is wise to do this in the callback function as it guarantees the attempt to subscribe happens only after the client is connected to the broker.
The on_message function prints the received message when it comes in, as well as the topic it was published under.
In the body of the code, we:
- Instantiate a client object with the client ID digi_mqtt_test.
- Define the callback functions to use upon connection and upon message receipt.
- Connect to an MQTT broker at m2m.eclipse.org, on port 1883 (the default MQTT port, or 8883 for MQTT over TLS) with a keepalive of 60 seconds (this is how often the client pings the broker to keep the connection alive).
The last line starts a network daemon that runs in the background and handles data transactions and messages, as well as keeping the socket open, until the script ends.
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