How to fix Python argument error

Question

I have a little python script I got from here. It listens for MQTT messages and plays a .wav file when a topic is published to. However, I get this error when the script is run:

TypeError: on_connect() takes exactly 3 arguments (4 given)

Where is the above error coming from? More importantly, how can I fix it?

Here's my full code:

import paho.mqtt.client as MQTT

# Don't forget to change the variables for the MQTT broker!
mqtt_topic = "button"
mqtt_broker_ip = "192.168.0.55"

song_path = '/home/pi/alerts/buzzer_alert.wav'
# change it to desire duration in seconds
duration_time = 5

client = mqtt.Client()

# These functions handle what happens when the MQTT client connects
# to the broker, and what happens then the topic receives a message
def on_connect(client, userdata, rc):
    # rc is the error code returned when connecting to the broker
    print "Connected!", str(rc)

    # Once the client has connected to the broker, subscribe to the topic
    client.subscribe(mqtt_topic)

def on_message(client, userdata, msg):
    # This function is called everytime the topic is published to.
    # If you want to check each message, and do something depending on
    # the content, the code to do this should be run in this function

    print "Topic: ", msg.topic + "\nMessage: " + str(msg.payload)

    # The message itself is stored in the msg variable
    # and details about who sent it are stored in userdata
    os.system('aplay -d {} {}'.format(duration_time, song_path))


# Here, we are telling the client which functions are to be run
# on connecting, and on receiving a message
client.on_connect = on_connect
client.on_message = on_message

# Once everything has been set up, we can (finally) connect to the broker
# 1883 is the listener port that the MQTT broker is using
client.connect(mqtt_broker_ip, 1883)

# Once we have told the client to connect, let the client object run itself
client.loop_forever()
client.disconnect()

Answer 1

change your on_connect callback to this definition:

def on_connect(client, userdata, flags, rc):
    # rc is the error code returned when connecting to the broker
    print "Connected!", str(rc)

    # Once the client has connected to the broker, subscribe to the topic
    client.subscribe(mqtt_topic)

as mentioned in the paho documentation:

@on_connect.setter
    def on_connect(self, func):
        """ Define the connect callback implementation.

        Expected signature is:
            connect_callback(client, userdata, flags, rc)

        client:     the client instance for this callback
        userdata:   the private user data as set in Client() or userdata_set()
        flags:      response flags sent by the broker
        rc:         the connection result