Great! We have a Service Server and we are even able to call it from the terminal. Now, let us create a python service client so we can call this new service from the code.

w/o OOP

We create a new file called add_two_ints_client_no_oop.py file in our package. What we want is, once we run it, it then sends the request to the service and then shuts down. That is, we will not spin the node. So, excluding OOP capabilities (ie, without creating a class) we start with the following code —

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

from example_interfaces.srv import AddTwoInts

def main(args=None):
    rclpy.init(args=args)
    node = Node("add_two_ints_client_no_oop")
    
    rclpy.shutdown()

if __name__ == "__main__":
    main()

The first thing that we do is, create a client. We do that simply by

client = node.create_client(AddTwoInts, "add_two_ints")

The two arguments are as —

1. srv_type
2. srv_name

And now we will be able to create a request, and send the request to the server. But before we do that, let us say that we run the client and the server node is not up yet. Then, if we send a request, it will fail. Alternative? Wait for the server.

We have a function called client.wait_for_service() that will wait indefinitely (if no argument given), then exit OR will stay in the while loop, where we can log the data. Look at the code snippet below —

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

from example_interfaces.srv import AddTwoInts

def main(args=None):
    rclpy.init(args=args)
    node = Node("add_two_ints_client_no_oop")

    client = node.create_client(AddTwoInts, "add_two_ints")
    while not client.wait_for_service(1.0): # wait for 1 second
        node.get_logger().warn("Waiting for server Add Two Ints...")
    
    rclpy.shutdown()

if __name__ == "__main__":
    main()