Compose two components into a single process in python

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The tutorial describes how to do this in C++. How do I do the same in python?

My attempt so far:

class NodeletA(Node):            
    def __init__(self):
        super().__init__('nodelet_a')

class NodeletB(Node):            
    def __init__(self):
        super().__init__('nodelet_b')

def main():   

    rclpy.init(args=None)
    executor = MultiThreadedExecutor()

    a = NodeletA()
    executor.add_node(a)

    b = NodeletB()
    executor.add_node(b)

    executor.spin()

    a.destroy_node()
    b.destroy_node()

    rclpy.shutdown()

This creates the components, but the spin method does not block like the rclpy.spin() does. The program instantly destroys the nodes and shuts down. how do I make executor.spin() blocking. The executor needs to be multithreaded because one of my nodes hosts an action server, and I need the execute_callback of the action to be non-blocking.

Thanks.