How do I concatenate lists in Python across multiple processes?

In Python, you can concatenate lists across multiple processes using the `multiprocessing` module. Here's a simple example that demonstrates how to achieve this using the `Manager` class, which allows shared lists among different processes.

import multiprocessing

def concatenate_lists(list1, list2, return_list):
    return_list.extend(list1)
    return_list.extend(list2)

if __name__ == '__main__':
    manager = multiprocessing.Manager()
    return_list = manager.list()  # Create a list that can be shared between processes

    list1 = [1, 2, 3]
    list2 = [4, 5, 6]

    p = multiprocessing.Process(target=concatenate_lists, args=(list1, list2, return_list))
    p.start()
    p.join()

    print("Concatenated List: ", list(return_list))  # Convert return_list to a normal list for output