rclpy: Node Basics

Most nodes have publishers and subscribers, both of which are creating by calling functions of the Node instance:

import rclpy
from rclpy.node import Node

from std_msgs.msg import String

class MyNode(Node):

    def __init__(self):
        super().__init__('my_node_name')

        self.publisher = self.create_publisher(String, 'output_topic', 10)
        self.subscription = self.create_subscription(
            String,
            'input_topic',
            self.callback,
            10)

    def callback(self, msg):
        self.get_logger().info("Recieved: %s" % msg.data)
        self.publisher.publish(msg)

if __name__ == "__main__":
    rclpy.init()
    my_node = MyNode()
    rclpy.spin(my_node)
    my_node.destroy_node()  # cleans up pub-subs, etc
    rclpy.shutdown()

Shutdown Hooks

ROS 1 had rospy.on_shutdown() - but there is not an equivalent in ROS 2. It really is not needed though, since we manually shut things down rather than was the case in rospy which used many global variables:

try:
    rclpy.spin(my_node)
except KeyboardInterrupt:
    pass
finally:
    my_node.on_shutdown()  # do any custom cleanup
    my_node.destroy_node()
    rclpy.shutdown()