ROS 2 Python Subscriber

A ROS 2 publisher sends messages to a topic, while a subscriber listens to the same topic and receives those messages. When a new message is published, the subscriber receives it and prints the message in the terminal.

ROS 2 also allows publishers and subscribers written in different programming languages, such as Python and C++, to communicate through the same topic.

Create one file

Create one file inside bumperbot_py_examples folder

simple_subscriber.py

Code for simple_subscriber.py

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

class SimpleSubscriber(Node):
    def __init__(self):
        super().__init__("subscriber")

        self.sub_ = self.create_subscription(String, "chatter", self.msgCallback, 10)

    def msgCallback(self, msg):
        self.get_logger().info("I heard: %s" % msg.data)

def main():
    rclpy.init()
    simple_subscriber = SimpleSubscriber()

    rclpy.spin(simple_subscriber)

    simple_subscriber.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':

    main()

Explanation for simple_subscriber.py

Register Node in setup.py

'simple_subscriber = bumperbot_py_examples.simple_subscriber:main'

Build the Workspace

cd bumperbot_ws

colcon build

Source the workspace & Run the Node

. install/setup.bash

ros2 run bumperbot_py_examples simple_subscriber

Publish a message

ros2 topic pub /chatter std_msgs/msg/String "data: 'hello'"

As shown in the image, we first published a message from one terminal using the publisher node. In a second terminal, we ran the subscriber node. The subscriber successfully received and displayed the published message in the subscriber terminal.

/chatter is the topic name.

Think of a ROS 2 topic as a communication channel.

/chatter
   ↑
 Topic name

• /chatter → where the message is sent
• std_msgs/msg/String → type/format of the message
• "data: 'hello'" → actual message

So this command means:

Publish the string "hello" to the /chatter topic.

Run Publishing Node

. install/setup.bash

ros2 run bumperbot_py_examples simple_publisher

As shown in the image, when we run the publisher node, the subscriber node automatically starts receiving and printing the published messages in the subscriber terminal.