Subscribing to control topics in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the MQTT client library.

Raspberry Pi

import [1]

Drag options to blanks, or click blank then click option'

Apaho.mqtt.client

Bmqtt.client

Cmqtt

Dpaho.mqtt

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'mqtt' or 'mqtt.client' which are not valid imports.

Forgetting the full path 'paho.mqtt.client'.

The correct import for the MQTT client in Python is paho.mqtt.client.

2fill in blank

medium

Complete the code to create a new MQTT client instance named 'client'.

Raspberry Pi

client = [1]()

Drag options to blanks, or click blank then click option'

Apaho.mqtt.Client

Bmqtt.Client

Cpaho.mqtt.client.Client

Dpaho.Client

Attempts:

3 left

💡 Hint

Common Mistakes

Using incomplete paths like 'mqtt.Client' or 'paho.Client'.

Forgetting to call the constructor with parentheses.

The MQTT client is created by calling paho.mqtt.client.Client().

3fill in blank

hard

Fix the error in the code to subscribe to the topic 'control/led'.

Raspberry Pi

client.subscribe([1])

Drag options to blanks, or click blank then click option'

A"control/led"

Bcontrol/led

C'control/led'

Dcontrol_led

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the topic string.

Using an incorrect topic name like 'control_led'.

The topic name must be a string, so it needs quotes like 'control/led'.

4fill in blank

hard

Fill both blanks to define a callback function that prints the received message payload.

Raspberry Pi

def on_message(client, userdata, [1]):
    print('Received:', [2].payload.decode())

Drag options to blanks, or click blank then click option'

Amsg

Bmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and inside the function.

Using undefined variable names.

The message parameter is commonly named msg. Use the same name to access the payload.

5fill in blank

hard

Fill all three blanks to connect to the broker, set the message callback, and start the loop.

Raspberry Pi

client.connect([1])
client.[2] = on_message
client.[3]()

Drag options to blanks, or click blank then click option'

A'localhost'

Bon_message

Cloop_start

Dloop_forever

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong broker address.

Not assigning the callback correctly.

Using loop_forever() instead of loop_start() when asynchronous loop is needed.

Connect to the broker at 'localhost', assign the on_message callback, and start the loop with loop_start().

Practice

1. What is the main purpose of subscribing to control topics on a Raspberry Pi using MQTT?

easy

A. To send commands from the Raspberry Pi to other devices

B. To update the Raspberry Pi operating system automatically

C. To store data locally on the Raspberry Pi

D. To listen and react to control commands sent to the Raspberry Pi

Solution

1. Step 1: Understand MQTT subscription

   Subscribing means the Raspberry Pi listens to messages on a topic.

2. Step 2: Identify the purpose of control topics

   Control topics send commands that the Raspberry Pi should react to.

3. Final Answer:

   To listen and react to control commands sent to the Raspberry Pi -> Option D

4. Quick Check:

   Subscribing = Listening to commands [OK]

Hint: Subscribing means listening, not sending commands [OK]

Common Mistakes:

• Confusing subscribing with publishing
• Thinking subscription stores data
• Assuming it updates the OS

2. Which of the following is the correct Python syntax to subscribe to an MQTT topic named "home/lights/control" using the Paho MQTT client?

easy

A. client.connect("home/lights/control")

B. client.sub("home/lights/control")

C. client.subscribe("home/lights/control")

D. client.listen("home/lights/control")

Solution

1. Step 1: Recall Paho MQTT subscribe method

   The method to subscribe to a topic is subscribe().

2. Step 2: Match the correct syntax

   Using client.subscribe("home/lights/control") is correct syntax.

3. Final Answer:

   client.subscribe("home/lights/control") -> Option C

4. Quick Check:

   Subscribe method is subscribe() [OK]

Hint: Subscribe uses .subscribe(), not .sub() or .listen() [OK]

Common Mistakes:

• Using .sub() instead of .subscribe()
• Confusing subscribe with connect
• Using non-existent methods like .listen()

3. Given the following code snippet, what will be printed when a message with payload "ON" is received on the topic "home/lights/control"?

def on_message(client, userdata, message):
    if message.payload.decode() == "ON":
        print("Turning lights ON")
    else:
        print("Turning lights OFF")

client.subscribe("home/lights/control")
client.on_message = on_message

medium

A. Turning lights ON

B. Turning lights OFF

C. No output

D. Error: decode() not found

Solution

1. Step 1: Decode the message payload

   The payload is decoded to string and compared to "ON".

2. Step 2: Check the condition and print output

   Since payload is "ON", it prints "Turning lights ON".

3. Final Answer:

   Turning lights ON -> Option A

4. Quick Check:

   Payload "ON" triggers print "Turning lights ON" [OK]

Hint: Decode payload before comparing strings [OK]

Common Mistakes:

• Not decoding payload before comparison
• Assuming no output without explicit print
• Thinking decode() causes error

4. Identify the error in this MQTT subscription code snippet for Raspberry Pi:

def on_message(client, userdata, message):
    print(message.payload)

client.on_message = on_message
client.subscribe("home/lights/control")
client.loop_start()

medium

A. Incorrect callback function name

B. Missing client.connect() before subscribing

C. Using loop_start() instead of loop_forever()

D. No error, code is correct

Solution

1. Step 1: Check connection setup

   Before subscribing, the client must connect to the MQTT broker.

2. Step 2: Identify missing connect call

   The code lacks client.connect() call, so subscription won't work.

3. Final Answer:

   Missing client.connect() before subscribing -> Option B

4. Quick Check:

   Must connect before subscribe [OK]

Hint: Always connect client before subscribing [OK]

Common Mistakes:

• Skipping client.connect() call
• Confusing callback function names
• Thinking loop_start() is wrong here

5. You want your Raspberry Pi to subscribe to multiple control topics: "home/lights/control" and "home/thermostat/control". Which code snippet correctly subscribes to both and handles messages differently based on the topic?

hard

A. client.subscribe(["home/lights/control", "home/thermostat/control"])
def on_message(client, userdata, message):
topic = message.topic
if topic == "home/lights/control":
print("Light control received")
elif topic == "home/thermostat/control":
print("Thermostat control received")
client.on_message = on_message

B. client.subscribe("home/lights/control")
client.subscribe("home/thermostat/control")
def on_message(client, userdata, message):
if message.payload == "light":
print("Light control")
else:
print("Thermostat control")
client.on_message = on_message

C. client.subscribe("home/lights/control", "home/thermostat/control")
def on_message(client, userdata, message):
print("Control received")
client.on_message = on_message

D. client.subscribe("home/lights/control")
def on_message(client, userdata, message):
print("Control received")
client.on_message = on_message
client.subscribe("home/thermostat/control")

Solution

1. Step 1: Subscribe to multiple topics correctly

   client.subscribe(["home/lights/control", "home/thermostat/control"]) def on_message(client, userdata, message): topic = message.topic if topic == "home/lights/control": print("Light control received") elif topic == "home/thermostat/control": print("Thermostat control received") client.on_message = on_message uses a list to subscribe to both topics at once, which is valid.

2. Step 2: Handle messages based on topic

   The on_message function checks message.topic to differentiate actions.

3. Final Answer:

   correctly subscribes to both topics and handles them separately -> Option A

4. Quick Check:

   Subscribe list + topic check = correct [OK]

Hint: Subscribe with list and check message.topic [OK]

Common Mistakes:

• Passing multiple topics as separate arguments
• Checking payload instead of topic for routing
• Subscribing after setting callback incorrectly