Writing Python Subscriber in ROS2

There are three ways to to write a Python publisher in ROS2, namely:

• Old-school approach
• Object-oriented (member-function) approach
• Local function (lambda) approach

Below is an example of each approach to write a Python node listening to “Hello World” stream. The code is taken from here: https://github.com/ros2/examples/tree/foxy/rclpy/topics/minimal_subscriber/examples_rclpy_minimal_subscriber

Old-school approach:

import rclpy

from std_msgs.msg import String

g_node = None


def chatter_callback(msg):
    global g_node
    g_node.get_logger().info(
        'I heard: "%s"' % msg.data)


def main(args=None):
    global g_node
    rclpy.init(args=args)

    g_node = rclpy.create_node('minimal_subscriber')

    subscription = g_node.create_subscription(String, 'topic', chatter_callback, 10)
    subscription  # prevent unused variable warning

    while rclpy.ok():
        rclpy.spin_once(g_node)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    g_node.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Object-oriented (member-function) approach:

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('I heard: "%s"' % msg.data)


def main(args=None):
    rclpy.init(args=args)

    minimal_subscriber = MinimalSubscriber()

    rclpy.spin(minimal_subscriber)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    minimal_subscriber.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Local function (lambda) approach:

import rclpy

from std_msgs.msg import String


def main(args=None):
    rclpy.init(args=args)

    node = rclpy.create_node('minimal_subscriber')

    subscription = node.create_subscription(
        String, 'topic', lambda msg: node.get_logger().info('I heard: "%s"' % msg.data), 10)
    subscription  # prevent unused variable warning

    rclpy.spin(node)

    # Destroy the node explicitly
    # (optional - otherwise it will be done automatically
    # when the garbage collector destroys the node object)
    node.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()