Subscribing to control topics in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MQTT subscription callback?

Consider a Raspberry Pi subscribing to an MQTT topic home/lights/control. The callback prints the received message payload.

What will be printed when the message payload is ON?

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("broker.hivemq.com", 1883, 60)
client.subscribe("home/lights/control")

# Simulate receiving a message
class Msg:
    payload = b"ON"

on_message(client, None, Msg())

A Received message: ON

B Received message: b'ON'

C Received message: None

D Received message: on

💡 Hint

Remember that MQTT message payloads are bytes and need decoding.

The msg.payload is a bytes object. Using decode() converts it to a string. So the printed message is exactly Received message: ON.

🧠 Conceptual

intermediate

Which MQTT topic subscription pattern matches all control topics under home?

You want your Raspberry Pi to subscribe to all control topics under home, such as home/lights/control and home/thermostat/control. Which subscription topic pattern should you use?

A home/#

B home/+/+

C home/control/+

D home/+/control

💡 Hint

The plus sign + matches exactly one topic level.

The pattern home/+/control matches topics with exactly three levels where the middle level can be anything, like home/lights/control. The # wildcard matches all subtopics but would include unwanted topics.

🔧 Debug

advanced

Why does this MQTT subscription callback not print messages?

The Raspberry Pi subscribes to home/lights/control but the callback never prints any message. What is the problem?

Raspberry Pi

import paho.mqtt.client as mqtt

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

client = mqtt.Client()
client.on_message = on_message
client.connect("broker.hivemq.com", 1883, 60)
client.subscribe("home/lights/control")

# Missing client.loop_forever() or client.loop_start() here

A The client does not process network events because the loop is not started

B The callback function is missing a return statement

C The client.connect() call is missing the port number

D The subscription topic is incorrect and does not match any published messages

💡 Hint

MQTT clients need to run a loop to receive messages.

Without calling client.loop_start() or client.loop_forever(), the client does not listen for incoming messages, so the callback is never triggered.

📝 Syntax

advanced

Which option correctly subscribes to multiple topics in one call?

You want to subscribe your Raspberry Pi MQTT client to home/lights/control and home/thermostat/control in one call. Which code is correct?

A client.subscribe("home/lights/control", "home/thermostat/control")

B client.subscribe([("home/lights/control", 0), ("home/thermostat/control", 0)])

C client.subscribe({"home/lights/control":0, "home/thermostat/control":0})

D client.subscribe("home/lights/control", qos=0); client.subscribe("home/thermostat/control", qos=0)

💡 Hint

Refer to the Paho MQTT subscribe method for multiple topics.

The subscribe() method accepts a list of tuples for multiple topics with QoS. Option B uses the correct syntax. Option B is invalid because subscribe expects one topic or a list. Option B uses a dict which is not accepted. Option B calls subscribe twice, not one call.

🚀 Application

expert

How many messages will be received with this subscription pattern?

Your Raspberry Pi subscribes to home/+/control. The broker publishes messages to these topics:

• home/lights/control
• home/thermostat/control
• home/lights/status
• home/garden/control
• home/lights/control/extra

How many messages will your client receive?

A 2

B 4

C 3

D 5

💡 Hint

The plus sign + matches exactly one topic level.

The subscription home/+/control matches topics with exactly three levels where the middle level can be anything. The matching topics are home/lights/control, home/thermostat/control, and home/garden/control. The topic home/lights/status does not match because last level is not 'control'. The topic home/lights/control/extra has four levels and does not match.