Creating a subscriber node in ROS - Quick Revision & Summary

Recall & Review

beginner

What is a subscriber node in ROS?

A subscriber node listens to messages published on a topic and processes the incoming data.

beginner

Which function is used to create a subscriber in a ROS node?

The function rospy.Subscriber() is used to create a subscriber that listens to a specific topic.

intermediate

What are the main arguments required by rospy.Subscriber()?

The main arguments are: the topic name (string), the message type (e.g., std_msgs.msg.String), and the callback function to process messages.

beginner

Why do we need a callback function in a subscriber node?

The callback function is called automatically whenever a new message arrives on the subscribed topic. It defines how to handle the incoming data.

beginner

What is the purpose of rospy.spin() in a subscriber node?

rospy.spin() keeps the node running and listening for messages until the node is shut down.

Which of the following is NOT needed to create a subscriber node in ROS?

A Topic name

B Callback function

C Publisher object

D Message type

A publisher object is not needed to create a subscriber node. Subscribers only need the topic name, message type, and callback function.

What does the callback function in a subscriber node do?

A Publishes messages to a topic

B Processes incoming messages

C Initializes the ROS node

D Shuts down the node

The callback function processes incoming messages received on the subscribed topic.

Which ROS Python function keeps the subscriber node alive to keep receiving messages?

A rospy.spin()

B rospy.init_node()

C rospy.Publisher()

D rospy.shutdown()

rospy.spin() keeps the node running and listening for messages.

In ROS, what type of object is passed as the second argument to rospy.Subscriber()?

A Message type class

B Topic name string

C Callback function

D Node handle

The second argument is the message type class, such as std_msgs.msg.String.

What happens if you do not call rospy.spin() in a subscriber node?

A The callback function will be called repeatedly

B The node will not initialize

C The node will publish messages instead

D The node will exit immediately after starting

Without rospy.spin(), the node will exit immediately and stop listening for messages.

Explain how to create a subscriber node in ROS using Python.

Think about the steps from starting the node to listening for messages.

Describe the role of the callback function in a ROS subscriber node.

Focus on what happens when new data comes in.

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