Creating a subscriber node in ROS - Why You Should Know This

The Big Idea

Discover how your robot can listen and react instantly without messy code!

The Scenario

Imagine you want your robot to listen to sensor data and react instantly, but you have to manually check the sensor every moment in your code.

The Problem

Manually polling sensor data wastes time and CPU power, can miss important updates, and makes your code complex and hard to maintain.

The Solution

Creating a subscriber node lets your robot automatically receive and respond to messages as soon as they arrive, keeping your code clean and efficient.

Before vs After

✗ Before

while True:
    data = get_sensor_data()
    process(data)
    sleep(0.1)

✓ After

def callback(msg):
    process(msg.data)
subscriber = create_subscriber('sensor_topic', callback)

What It Enables

This lets your robot react in real-time to changing data without wasting resources or complicating your code.

Real Life Example

A drone automatically adjusting its flight path by listening to GPS updates published on a topic.

Key Takeaways

Manual polling is inefficient and error-prone.

Subscriber nodes receive data automatically when available.

This approach simplifies code and improves responsiveness.

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