paho-mqtt library usage in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MQTT client connection code?

Consider this Python code using paho-mqtt to connect to a broker and print connection status.

What will be printed when the client successfully connects?

Raspberry Pi

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, rc):
    if rc == 0:
        print("Connected successfully")
    else:
        print(f"Connection failed with code {rc}")

client = mqtt.Client()
client.on_connect = on_connect
client.connect("test.mosquitto.org", 1883, 60)
client.loop_start()

A Connected successfully

B Connection failed with code 1

C No output

D SyntaxError

💡 Hint

Check the meaning of rc == 0 in the on_connect callback.

When the client connects successfully, the return code rc is 0, so it prints "Connected successfully".

❓ Predict Output

intermediate

What message does this MQTT subscriber print when a message arrives?

This code subscribes to topic home/temperature and prints the message payload.

What will be printed if a message with payload 25.5 arrives?

Raspberry Pi

import paho.mqtt.client as mqtt

def on_message(client, userdata, msg):
    print(f"Received message: {msg.payload.decode()}")

client = mqtt.Client()
client.on_message = on_message
client.connect("test.mosquitto.org", 1883, 60)
client.subscribe("home/temperature")
client.loop_start()

A Received message: 25.5

B Received message: b'25.5'

C Received message: None

D TypeError

💡 Hint

Remember to decode the message payload from bytes to string.

The payload is bytes, so decoding it to string prints the actual message content.

🔧 Debug

advanced

Why does this MQTT client fail to receive messages after subscribing?

Look at this code snippet. The client subscribes but never receives messages. What is the problem?

Raspberry Pi

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("test.mosquitto.org", 1883, 60)
client.subscribe("home/livingroom")
# Missing loop_start or loop_forever call

A The topic name is invalid and causes subscription failure.

B The client never starts the network loop to process incoming messages.

C The client.connect call is missing a callback function.

D The client.subscribe call must be before connect.

💡 Hint

Check if the client processes network events after subscribing.

The client must run loop_start() or loop_forever() to handle incoming messages. Without it, messages are not received.

📝 Syntax

advanced

Which option correctly publishes a message with QoS 1 using paho-mqtt?

Choose the code snippet that publishes the message "Hello" to topic test/topic with QoS level 1.

A client.publish("test/topic", "Hello", qos=2)

B client.publish(topic="test/topic", message="Hello", qos=1)

C client.publish("test/topic", payload="Hello", qos=1)

D client.publish("test/topic", payload="Hello", qos=3)

💡 Hint

Check the correct parameter names and valid QoS values (0, 1, or 2).

The publish method uses payload for message and QoS must be 0, 1, or 2. Option C uses correct parameters and QoS 1.

🚀 Application

expert

How many messages will be received by this MQTT client after publishing?

This client subscribes to sensor/# and publishes to sensor/temperature. How many messages will the client receive?

Raspberry Pi

import paho.mqtt.client as mqtt

received = []

def on_message(client, userdata, msg):
    received.append(msg.topic)

client = mqtt.Client()
client.on_message = on_message
client.connect("test.mosquitto.org", 1883, 60)
client.subscribe("sensor/#")
client.loop_start()

client.publish("sensor/temperature", "22.5")

import time
time.sleep(1)  # wait for message

client.loop_stop()

print(len(received))

A Raises RuntimeError

B 0

C 2

D 1

💡 Hint

Consider if the client receives its own published messages on subscribed topics.

The client subscribes to sensor/#, which matches sensor/temperature. The client receives its own published message, so received has one entry.