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.

Practice

1. What is the main purpose of the paho-mqtt library on a Raspberry Pi?

easy

A. To send and receive messages using the MQTT protocol

B. To control the Raspberry Pi's GPIO pins

C. To create a web server on the Raspberry Pi

D. To manage file storage on the Raspberry Pi

Solution

1. Step 1: Understand MQTT protocol usage

   MQTT is a messaging protocol used for sending and receiving messages between devices.

2. Step 2: Identify paho-mqtt library role

   The paho-mqtt library allows Raspberry Pi to connect to MQTT brokers to publish and subscribe messages.

3. Final Answer:

   To send and receive messages using the MQTT protocol -> Option A

4. Quick Check:

   paho-mqtt = MQTT messaging [OK]

Hint: Remember: paho-mqtt is for MQTT messaging only [OK]

Common Mistakes:

• Confusing paho-mqtt with GPIO control libraries
• Thinking it manages web servers
• Assuming it handles file storage

2. Which of the following is the correct way to import the paho-mqtt client in Python?

easy

A. import mqtt.paho.client

B. import paho_mqtt_client as mqtt

C. import paho.mqtt.client as mqtt

D. import paho.mqtt as client

Solution

1. Step 1: Recall correct import syntax

   The official paho-mqtt Python client is imported as import paho.mqtt.client as mqtt.

2. Step 2: Check other options

   Other options use incorrect module names or syntax.

3. Final Answer:

   import paho.mqtt.client as mqtt -> Option C

4. Quick Check:

   Correct import = import paho.mqtt.client as mqtt [OK]

Hint: Use dots for module import, underscores are wrong here [OK]

Common Mistakes:

• Using underscores instead of dots in import
• Swapping module parts order
• Importing incomplete module path

3. What will the following code print when a message is received on topic 'test/topic'?

import paho.mqtt.client as mqtt

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

client = mqtt.Client()
client.on_message = on_message
client.connect('broker.hivemq.com', 1883)
client.subscribe('test/topic')
client.loop_start()

# Assume a message with payload 'Hello' is published to 'test/topic'

medium

A. Received b'Hello' on topic 'test/topic'

B. Received 'Hello' on topic 'test/topic'

C. Received 'Hello' on topic 'broker.hivemq.com'

D. No output because loop_start() is missing

Solution

1. Step 1: Understand on_message callback

   The callback decodes the message payload bytes to string and prints it with the topic.

2. Step 2: Check code behavior on receiving 'Hello'

   The payload 'Hello' is decoded and printed with the subscribed topic 'test/topic'.

3. Final Answer:

   Received 'Hello' on topic 'test/topic' -> Option B

4. Quick Check:

   Decoded payload + topic = Received 'Hello' on topic 'test/topic' [OK]

Hint: Decode payload bytes to string before printing [OK]

Common Mistakes:

• Printing raw bytes without decoding
• Confusing topic with broker address
• Forgetting to start the loop

4. Identify the error in this MQTT client setup code:

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
client.subscribe('home/temperature')
client.on_message = lambda c, u, m: print(m.payload)
# Missing client.loop_forever()

medium

A. The on_message callback is assigned after subscribe()

B. The subscribe topic is invalid

C. The broker address is incorrect

D. The client loop method to process network events is missing

Solution

1. Step 1: Check client loop usage

   MQTT client needs a loop method like loop_forever() or loop_start() to process messages.

2. Step 2: Verify other parts

   Assigning on_message after subscribe is allowed; broker and topic are valid.

3. Final Answer:

   The client loop method to process network events is missing -> Option D

4. Quick Check:

   Missing loop = no message processing [OK]

Hint: Always run client.loop_start() or loop_forever() after setup [OK]

Common Mistakes:

• Not running the client loop to handle messages
• Thinking on_message must be set before subscribe
• Assuming broker or topic is wrong without checking

5. You want your Raspberry Pi to publish temperature data every 10 seconds to topic 'home/temperature' using paho-mqtt. Which code snippet correctly implements this with proper connection, publishing, and loop handling? A)

import paho.mqtt.client as mqtt
import time

client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)

while True:
    client.publish('home/temperature', '22.5')
    time.sleep(10)

B)

import paho.mqtt.client as mqtt
import time

client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
client.loop_start()

while True:
    client.publish('home/temperature', '22.5')
    time.sleep(10)

C)

import paho.mqtt.client as mqtt
import time

client = mqtt.Client()
client.loop_start()
client.connect('broker.hivemq.com', 1883)

while True:
    client.publish('home/temperature', '22.5')
    time.sleep(10)

D)

import paho.mqtt.client as mqtt
import time

client = mqtt.Client()
client.connect('broker.hivemq.com', 1883)
client.loop_forever()

while True:
    client.publish('home/temperature', '22.5')
    time.sleep(10)

hard

A. Connect, start loop, then publish in a loop

B. Connect and publish without starting the loop

C. Start loop before connecting, then publish

D. Use loop_forever() which blocks before publishing

Solution

1. Step 1: Understand client loop and connection order

   Client must connect first, then start the network loop with loop_start() to handle background MQTT tasks.

2. Step 2: Check each option's order and behavior

   Connect and publish without starting the loop connects and publishes without starting the loop, missing network processing. Connect, start loop, then publish in a loop correctly connects, starts loop, then publishes repeatedly. Start loop before connecting, then publish starts loop before connect (incorrect order). Use loop_forever() which blocks before publishing blocks with loop_forever() preventing publishing.

3. Final Answer:

   Connect, start loop, then publish in a loop -> Option A

4. Quick Check:

   Connect -> loop_start() -> publish loop = Connect, start loop, then publish in a loop [OK]

Hint: Always connect before loop_start(), loop_forever() blocks code [OK]

Common Mistakes:

• Starting loop before connecting
• Not starting loop at all
• Using loop_forever() which blocks publishing code