Creating a publisher node in ROS - Try It Yourself

Creating a publisher node

📖 Scenario: You are building a simple ROS node that sends messages to other nodes. This is like a radio station broadcasting news to listeners. Your node will publish a greeting message repeatedly.

🎯 Goal: Create a ROS publisher node in Python that sends the message "Hello ROS" on the topic chatter at 1 message per second.

📋 What You'll Learn

Create a ROS node named talker

Create a publisher for the topic chatter with message type String

Set the publishing rate to 1 Hz

Publish the message "Hello ROS" repeatedly

💡 Why This Matters

🌍 Real World

Publishing messages is how ROS nodes communicate sensor data, commands, or status updates in robots.

💼 Career

Understanding how to create publisher nodes is essential for robotics software engineers working with ROS to build modular and interactive robot systems.

1

Setup ROS node and import dependencies

Write the first lines to import rclpy and String from std_msgs.msg. Then initialize the ROS client library with rclpy.init() and create a node named talker using rclpy.create_node('talker').

ROS

# Import rclpy and String
# Initialize ROS client library
# Create a node named 'talker'
# Your code here

Hint

Remember to import rclpy and the message type String. Then initialize ROS and create the node with the exact name 'talker'.

2

Create a publisher and set the publishing rate

Add a publisher to the node named publisher that publishes String messages on the topic chatter. Then create a rate variable set to 1.0 to set the publishing frequency to 1 Hz.

ROS

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

rclpy.init()
node = rclpy.create_node('talker')
# Create a publisher for 'chatter' topic
# Create a rate of 1 Hz
# Your code here

Hint

Use node.create_publisher(String, 'chatter') to create the publisher. Use rate = 1.0 to set 1 message per second.

3

Publish the message in a loop

Write a while rclpy.ok() loop that creates a String message with data 'Hello ROS' and publishes it using publisher.publish(). Then call rclpy.spin_once(node, timeout_sec=rate) inside the loop to wait for 1 second between messages and process ROS events.

ROS

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

rclpy.init()
node = rclpy.create_node('talker')
publisher = node.create_publisher(String, 'chatter')
rate = 1.0

# Publish 'Hello ROS' repeatedly
# Your code here

Hint

Use a while rclpy.ok() loop to keep publishing. Create a String message, set data to 'Hello ROS', publish it, then rclpy.spin_once(node, timeout_sec=rate).

4

Shutdown the ROS node cleanly

After the loop, call node.destroy_node() to clean up the node, then call rclpy.shutdown() to shut down the ROS client library.

ROS

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

rclpy.init()
node = rclpy.create_node('talker')
publisher = node.create_publisher(String, 'chatter')
rate = 1.0

while rclpy.ok():
    msg = String()
    msg.data = 'Hello ROS'
    publisher.publish(msg)
    rclpy.spin_once(node, timeout_sec=rate)

# Destroy the node and shutdown rclpy
# Your code here

Hint

Call node.destroy_node() and then rclpy.shutdown() after the publishing loop to clean up.

Practice

1. What is the main purpose of a publisher node in ROS?

easy

A. To store data in a database

B. To receive messages from other nodes

C. To compile the ROS package

D. To send messages on a topic to share information

Solution

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

   A publisher node sends messages to other nodes by publishing on a topic.

2. Step 2: Compare options with the role

   Only To send messages on a topic to share information describes sending messages on a topic, which matches the publisher's role.

3. Final Answer:

   To send messages on a topic to share information -> Option D

4. Quick Check:

   Publisher node = sends messages [OK]

Hint: Publisher nodes send messages, subscribers receive them [OK]

Common Mistakes:

• Confusing publisher with subscriber
• Thinking publisher stores data
• Assuming publisher compiles code

2. Which of the following is the correct way to create a publisher in a ROS2 Python node?

easy

A. self.createPublisher(String, 'topic_name', 10)

B. self.create_publisher(String, 'topic_name', 10)

C. create_publisher(String, 'topic_name', 10)

D. self.create_publisher('topic_name', String, 10)

Solution

1. Step 1: Recall ROS2 Python publisher syntax

   The method is called on the node instance as self.create_publisher, with message type first, then topic name, then queue size.

2. Step 2: Check each option's syntax

   self.create_publisher(String, 'topic_name', 10) matches the correct method name and argument order. self.createPublisher(String, 'topic_name', 10) has wrong method name casing. create_publisher(String, 'topic_name', 10) misses self. self.create_publisher('topic_name', String, 10) swaps argument order.

3. Final Answer:

   self.create_publisher(String, 'topic_name', 10) -> Option B

4. Quick Check:

   Correct method and argument order = self.create_publisher(String, 'topic_name', 10) [OK]

Hint: Use self.create_publisher with message type first [OK]

Common Mistakes:

• Wrong method name casing
• Missing self when calling method
• Swapping topic name and message type

3. Given this ROS2 Python publisher code snippet, what will be printed when the timer callback runs?

def timer_callback(self):
    msg = String()
    msg.data = 'Hello ROS'
    self.publisher.publish(msg)
    print('Published:', msg.data)

medium

A. Published: Hello ROS

B. Hello ROS

C. Published: msg.data

D. Error: msg.data undefined

Solution

1. Step 1: Analyze the print statement

   The print outputs the string 'Published:' followed by the value of msg.data, which is 'Hello ROS'.

2. Step 2: Confirm msg.data value

   msg.data is assigned 'Hello ROS' before publishing, so the print shows 'Published: Hello ROS'.

3. Final Answer:

   Published: Hello ROS -> Option A

4. Quick Check:

   Print shows 'Published:' plus msg.data [OK]

Hint: Print uses string plus msg.data value [OK]

Common Mistakes:

• Printing variable name instead of value
• Assuming print shows only msg.data
• Confusing msg.data with undefined

4. Identify the error in this ROS2 Python publisher node snippet:

def __init__(self):
    self.publisher = self.create_publisher(String, 'chatter', 10)

def timer_callback(self):
    msg = String()
    msg.data = 'Hello'
    self.publisher.publish(msg)

self.timer = self.create_timer(1.0, timer_callback)

medium

A. timer_callback is missing 'self' parameter

B. create_publisher is called with wrong arguments

C. Timer is created outside the class constructor

D. msg.data is assigned incorrectly

Solution

1. Step 1: Check code structure and indentation

   The self.timer = self.create_timer(1.0, timer_callback) line lacks indentation and appears outside __init__ at class level, where 'self' is undefined, causing NameError.

2. Step 2: Verify other options

   A: timer_callback includes 'self'. B: create_publisher args are correct (type, topic, qos). D: msg.data = 'Hello' is valid.

3. Final Answer:

   Timer is created outside the class constructor -> Option C

4. Quick Check:

   self assignments must be inside instance methods like __init__ [OK]

Hint: Create timers inside __init__ with self.create_timer(1.0, self.timer_callback) [OK]

Common Mistakes:

• Omitting 'self' in method parameters
• Misordering create_publisher arguments
• Creating timer outside class context

5. You want to create a ROS2 publisher node that publishes integers incrementing every second starting from 0. Which code snippet correctly implements this behavior?

hard

A. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 0
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback(self):
msg = Int32()
msg.data = self.i
self.publisher.publish(msg)
self.i += 1

B. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 0
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback():
msg = Int32()
msg.data = self.i
self.publisher.publish(msg)
self.i += 1

C. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 0
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback(self):
msg = Int32()
msg.data = 0
self.publisher.publish(msg)
self.i += 1

D. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 1
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback(self):
msg = Int32()
msg.data = self.i
self.publisher.publish(msg)
self.i = 0

Solution

1. Step 1: Check timer_callback method signatures

   Options A and C define timer_callback(self); B omits 'self', causing AttributeError on self.i and self.publisher.

2. Step 2: Verify increment logic

   class IntPublisher(Node): def __init__(self): super().__init__('int_publisher') self.publisher = self.create_publisher(Int32, 'numbers', 10) self.i = 0 self.timer = self.create_timer(1.0, self.timer_callback) def timer_callback(self): msg = Int32() msg.data = self.i self.publisher.publish(msg) self.i += 1 initializes self.i=0 and does self.i += 1 after publishing, yielding 0,1,2,... class IntPublisher(Node): def __init__(self): super().__init__('int_publisher') self.publisher = self.create_publisher(Int32, 'numbers', 10) self.i = 0 self.timer = self.create_timer(1.0, self.timer_callback) def timer_callback(self): msg = Int32() msg.data = 0 self.publisher.publish(msg) self.i += 1 always publishes 0. class IntPublisher(Node): def __init__(self): super().__init__('int_publisher') self.publisher = self.create_publisher(Int32, 'numbers', 10) self.i = 1 self.timer = self.create_timer(1.0, self.timer_callback) def timer_callback(self): msg = Int32() msg.data = self.i self.publisher.publish(msg) self.i = 0 publishes 1 then sets self.i=0 repeatedly.

3. Final Answer:

   self.i = 0; msg.data = self.i; self.i += 1 -> Option A

4. Quick Check:

   self.i starts 0, increment after publish [OK]

Hint: Use self.i and increment after publishing in timer callback [OK]

Common Mistakes:

• Missing 'self' in timer_callback
• Not incrementing the counter
• Resetting counter inside callback