Creating Python Service in ROS2

Step 1: Create the service (.srv) file, namely ServiceName.srv, inside an “srv” folder.

Step 2: Modify setup.py.

Step 3: Modify package.xml.

Step 4: Write the service_server node in the package subfolder inside the package folder.

There are two ways to write a service_server node in Python: 1) old-school approach, and 2) member-function approach. The following is the examples of each approach, taken from here: https://github.com/ros2/examples/tree/foxy/rclpy/services/minimal_service

Old-school approach:

from example_interfaces.srv import AddTwoInts

import rclpy

g_node = None


def add_two_ints_callback(request, response):
    global g_node
    response.sum = request.a + request.b
    g_node.get_logger().info(
        'Incoming request\na: %d b: %d' % (request.a, request.b))

    return response


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

    g_node = rclpy.create_node('minimal_service')

    srv = g_node.create_service(AddTwoInts, 'add_two_ints', add_two_ints_callback)
    while rclpy.ok():
        rclpy.spin_once(g_node)

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


if __name__ == '__main__':
    main()

Member-function approach:

from example_interfaces.srv import AddTwoInts

import rclpy
from rclpy.node import Node


class MinimalService(Node):

    def __init__(self):
        super().__init__('minimal_service')
        self.srv = self.create_service(AddTwoInts, 'add_two_ints', self.add_two_ints_callback)

    def add_two_ints_callback(self, request, response):
        response.sum = request.a + request.b
        self.get_logger().info('Incoming request\na: %d b: %d' % (request.a, request.b))

        return response


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

    minimal_service = MinimalService()

    rclpy.spin(minimal_service)

    rclpy.shutdown()


if __name__ == '__main__':
    main()

Step 5: Write the service_client node in the package subfolder inside the package folder. The following is an example using the member-function approach, taken from here: https://github.com/ros2/examples/tree/foxy/rclpy/services/minimal_client

import sys

from example_interfaces.srv import AddTwoInts
import rclpy
from rclpy.node import Node


class MinimalClientAsync(Node):

    def __init__(self):
        super().__init__('minimal_client_async')
        self.cli = self.create_client(AddTwoInts, 'add_two_ints')
        while not self.cli.wait_for_service(timeout_sec=1.0):
            self.get_logger().info('service not available, waiting again...')
        self.req = AddTwoInts.Request()

    def send_request(self):
        self.req.a = int(sys.argv[1])
        self.req.b = int(sys.argv[2])
        self.future = self.cli.call_async(self.req)


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

    minimal_client = MinimalClientAsync()
    minimal_client.send_request()

    while rclpy.ok():
        rclpy.spin_once(minimal_client)
        if minimal_client.future.done():
            try:
                response = minimal_client.future.result()
            except Exception as e:
                minimal_client.get_logger().info(
                    'Service call failed %r' % (e,))
            else:
                minimal_client.get_logger().info(
                    'Result of add_two_ints: for %d + %d = %d' %
                    (minimal_client.req.a, minimal_client.req.b, response.sum))
            break

    minimal_client.destroy_node()
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Step 6: Build the package by using the “colcon build” command.