Creating a subscriber node in ROS - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this ROS subscriber node print when it receives a message?

Consider this ROS subscriber node written in Python. What will it print when a message with data 'Hello ROS' is received?

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class MinimalSubscriber(Node):
    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            String,
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning

    def listener_callback(self, msg):
        self.get_logger().info(f'Received: {msg.data}')


def main(args=None):
    rclpy.init(args=args)
    minimal_subscriber = MinimalSubscriber()
    rclpy.spin(minimal_subscriber)
    minimal_subscriber.destroy_node()
    rclpy.shutdown()

A It prints: 'Received: Hello ROS'

B It prints: 'Hello ROS'

C It prints: 'Received: std_msgs.msg.String(data=Hello ROS)'

D It prints nothing because the callback is not called

💡 Hint

Look at how the callback uses msg.data inside the info log.

The subscriber callback receives a message object of type String. Accessing msg.data gives the string content. The logger prints 'Received: ' followed by the message data.

📝 Syntax

intermediate

Which option correctly creates a ROS2 subscriber node in Python?

Select the code snippet that correctly creates a subscriber node for topic 'chatter' with message type String.

A self.subscription = self.create_subscription(String, 'chatter', self.callback, 10)

B self.subscription = create_subscription(String, 'chatter', self.callback, 10)

C self.subscription = self.create_subscription('String', 'chatter', self.callback, 10)

D self.subscription = self.create_subscription(String, 'chatter', callback, 10)

💡 Hint

Remember that create_subscription is a method of the Node class and requires the message type, topic name, callback, and queue size.

Option A correctly calls self.create_subscription with the message type String, topic 'chatter', the callback method, and queue size 10. Option A misses self. Option A passes a string instead of the message type. Option A uses callback without self.

❓ state_output

advanced

What is the value of 'count' after receiving 3 messages in this subscriber node?

Given this subscriber node that counts messages received, what is the value of self.count after 3 messages arrive?

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class CounterSubscriber(Node):
    def __init__(self):
        super().__init__('counter_subscriber')
        self.count = 0
        self.subscription = self.create_subscription(
            String,
            'count_topic',
            self.listener_callback,
            10)

    def listener_callback(self, msg):
        self.count += 1
        self.get_logger().info(f'Message count: {self.count}')

A 0

B 2

C 3

D Raises an AttributeError

💡 Hint

The count increments by 1 each time the callback runs.

The count starts at 0 and increments by 1 on each message received. After 3 messages, count is 3.

🔧 Debug

advanced

Why does this ROS2 subscriber node fail to receive messages?

Examine this subscriber node code. Why does it not receive any messages?

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class SilentSubscriber(Node):
    def __init__(self):
        super().__init__('silent_subscriber')
        self.subscription = self.create_subscription(
            String,
            'silent_topic',
            self.listener_callback,
            10)

    def listener_callback(self, msg):
        print(f'Received: {msg.data}')


def main(args=None):
    rclpy.init(args=args)
    node = SilentSubscriber()
    # Missing rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

A Because the subscription queue size 10 is too small

B Because the topic name 'silent_topic' is incorrect

C Because the callback uses print instead of get_logger().info()

D Because rclpy.spin(node) is missing, so callbacks never run

💡 Hint

Think about what keeps the node alive and processing callbacks.

Without calling rclpy.spin(node), the node does not enter the loop to process incoming messages and callbacks are never triggered.

🧠 Conceptual

expert

Which statement best describes the role of QoS (Quality of Service) in ROS2 subscriber nodes?

Choose the most accurate explanation of how QoS settings affect a ROS2 subscriber node.

A QoS is a debugging tool that logs message traffic for subscriber nodes

B QoS controls message delivery reliability, durability, and history, affecting how the subscriber receives messages under different network conditions

C QoS determines the message type the subscriber can accept and automatically converts incompatible messages

D QoS only sets the maximum queue size for incoming messages and does not affect delivery guarantees

💡 Hint

Think about how ROS2 handles message delivery in unreliable networks or when nodes start late.

QoS policies in ROS2 define how messages are delivered, including reliability (best effort or reliable), durability (transient or volatile), and history (keep last or all), which influence subscriber behavior.

Practice

1.

What is the main purpose of a subscriber node in ROS?

easy

A. To send messages to other nodes

B. To listen and react to messages published on a topic

C. To create new topics for communication

D. To store messages permanently

Solution

1. Step 1: Understand the role of a subscriber node

   A subscriber node listens to messages sent on a specific topic in ROS.

2. Step 2: Compare options with the subscriber role

   Only To listen and react to messages published on a topic describes listening and reacting to messages, which matches the subscriber's purpose.

3. Final Answer:

   To listen and react to messages published on a topic -> Option B

4. Quick Check:

   Subscriber node = listens to messages [OK]

Hint: Subscriber nodes always listen to topics, not send [OK]

Common Mistakes:

• Confusing subscriber with publisher
• Thinking subscriber creates topics
• Assuming subscriber stores messages permanently

2.

Which of the following is the correct way to create a subscriber in ROS 2 Python?

node.create_subscription(____, 'chatter', callback, 10)

easy

A. std_msgs.msg.String

B. String

C. std_msgs/String

D. string

Solution

1. Step 1: Identify the correct message type import

   In ROS 2 Python, message types are imported as modules, e.g., std_msgs.msg.String.

2. Step 2: Match the correct syntax for create_subscription

   The first argument must be the full message type class, so std_msgs.msg.String is correct.

3. Final Answer:

   std_msgs.msg.String -> Option A

4. Quick Check:

   Message type = std_msgs.msg.String [OK]

Hint: Use full message type path with .msg in ROS 2 Python [OK]

Common Mistakes:

• Using just 'String' without module path
• Using slash notation like 'std_msgs/String'
• Using lowercase 'string' instead of class name

3.

Given this ROS 2 Python subscriber code snippet, what will be printed when a message with data 'Hello' arrives?

def callback(msg):
    print(f'Received: {msg.data}')

node.create_subscription(std_msgs.msg.String, 'chatter', callback, 10)

medium

A. Received: msg.data

B. Hello

C. Received: Hello

D. msg.data

Solution

1. Step 1: Understand the callback function

   The callback prints 'Received: ' followed by the message's data attribute.

2. Step 2: Substitute the message data value

   When the message data is 'Hello', the print outputs 'Received: Hello'.

3. Final Answer:

   Received: Hello -> Option C

4. Quick Check:

   Prints 'Received: ' + msg.data [OK]

Hint: Callback prints 'Received: ' plus message data [OK]

Common Mistakes:

• Printing just the data without prefix
• Printing the variable name instead of its value
• Confusing string formatting syntax

4.

What is wrong with this ROS 2 Python subscriber code?

def callback(msg):
    print(msg.data)

node.create_subscription(std_msgs.msg.String, 'chatter', callback())

medium

A. Missing message type in create_subscription

B. Queue size argument is missing but required

C. Topic name should be a variable, not a string

D. Callback is called immediately instead of passed as a function

Solution

1. Step 1: Check how callback is passed

   The code uses callback() which calls the function immediately instead of passing the function reference.

2. Step 2: Understand correct callback usage

   We must pass callback without parentheses so it is called on message arrival.

3. Final Answer:

   Callback is called immediately instead of passed as a function -> Option D

4. Quick Check:

   Pass callback function, not call it [OK]

Hint: Pass callback without () to create_subscription [OK]

Common Mistakes:

• Adding parentheses to callback in subscription
• Omitting queue size argument (optional in some versions)
• Using variable instead of string for topic name

5.

You want to create a subscriber node that listens to the topic 'sensor_data' and prints only messages where the data is greater than 10. Which callback function correctly implements this?

def callback(msg):
    # Fill in the condition
    if ____:
        print(f"Data: {msg.data}")

hard

A. msg.data > 10

B. msg.data >= 10

C. msg.data < 10

D. msg.data == 10

Solution

1. Step 1: Understand the requirement

   We want to print messages only if data is greater than 10, so the condition must check msg.data > 10.

2. Step 2: Compare options with condition

   msg.data > 10 matches exactly 'greater than 10'. Others check equal or less, which do not meet the requirement.

3. Final Answer:

   msg.data > 10 -> Option A

4. Quick Check:

   Condition for data > 10 is msg.data > 10 [OK]

Hint: Use > 10 to filter messages with data above 10 [OK]

Common Mistakes:

• Using >= instead of > changes condition
• Using < or == prints wrong messages
• Not adding any condition prints all messages