By default, services in rclpy are asynchronous. For more info see this
How-to Guide.
from my_ros_package.srv import ServiceName
from threading import Thread
cli = my_node.create_client(ServiceName, 'service_name')
cli.wait_for_service()
# Need to spin before making the service call or we will deadlock
spin_thread = Thread(target=rclpy.spin, args=(my_node,))
spin_thread.start()
req = ServiceName.Request()