How to publish json data on mqtt broker in python?

Question

I am trying to publish JSON data on MQTT broker topic. this is the source code, I tried-

import paho.mqtt.client as mqtt
import json
# Define Variables
MQTT_HOST = "localhost"
MQTT_PORT = 1883
MQTT_KEEPALIVE_INTERVAL = 45
MQTT_TOPIC = "irisPayload"

MQTT_MSG=json.dumps({"sepalLength": "6.4","sepalWidth":  "3.2","petalLength": "4.5","petalWidth":  "1.5"});
# Define on_publish event function
def on_publish(client, userdata, mid):
    print "Message Published..."
# Initiate MQTT Client
mqttc = mqtt.Client()

# Register publish callback function
mqttc.on_publish = on_publish

# Connect with MQTT Broker
mqttc.connect(MQTT_HOST, MQTT_PORT, MQTT_KEEPALIVE_INTERVAL)        

# Publish message to MQTT Broker    
mqttc.publish(MQTT_TOPIC,MQTT_MSG)

# Disconnect from MQTT_Broker
mqttc.disconnect()

I just want to publish JSON data without payload={jsondata}format, how to remove payload as it comes every time when I am publishing my data on particular topic?

In the log it shows msg like this--- [payload={ "sepalLength": "6.4", "sepalWidth": "3.2", "petalLength": "4.5", "petalWidth": "1.5" }]

Answer 1

I add some modification for your code, then you can publish json as string, then receive string and convert to json.

Sample code:

import paho.mqtt.client as mqtt
import json
# Define Variables
MQTT_HOST = "localhost"
MQTT_PORT = 1883
MQTT_KEEPALIVE_INTERVAL = 45
MQTT_TOPIC = "irisPayload"

MQTT_MSG=json.dumps({"sepalLength": "6.4","sepalWidth":  "3.2","petalLength": "4.5","petalWidth":  "1.5"});
# Define on_publish event function
def on_publish(client, userdata, mid):
    print "Message Published..."

def on_connect(client, userdata, flags, rc):
    client.subscribe(MQTT_TOPIC)
    client.publish(MQTT_TOPIC, MQTT_MSG)

def on_message(client, userdata, msg):
    print(msg.topic)
    print(msg.payload) # <- do you mean this payload = {...} ?
    payload = json.loads(msg.payload) # you can use json.loads to convert string to json
    print(payload['sepalWidth']) # then you can check the value
    client.disconnect() # Got message then disconnect

# Initiate MQTT Client
mqttc = mqtt.Client()

# Register publish callback function
mqttc.on_publish = on_publish
mqttc.on_connect = on_connect
mqttc.on_message = on_message

# Connect with MQTT Broker
mqttc.connect(MQTT_HOST, MQTT_PORT, MQTT_KEEPALIVE_INTERVAL)

# Loop forever
mqttc.loop_forever()