[FIXED] how can I disable multiple instances of a python script?

Issue

I made a python program and I want to disable the option to launch it multiple times in parallel.

I searched the web and found that I need to do it with socket and ports, but I didn't find any example of how to do it(I know the steps, but I need to search every step on google and I am not so sure how to start).
I also found that there might be libraries that can do that but to specific operating systems. My script only targets Windows 10, so I need to find a library that does that to Windows if there is one.
Do you know such libraries or can you give a more detailed example of how to make this socket listener?
The results that I found on the web were actually just one question in python 2.7 (I am using 3.8): https://stackoverflow.com/a/24381532/13786031

Solution

I'm not sure if it's possible to reliably detect if one instance (process) of the application is already running, but one possible way to do this is to reserve a TCP port for the program. The first application that starts will listen on that port and act like the server. Since only process can listen at the same port at the same time, all following processes will fail to create a listen socket and could then act like a client.

You could use a multiprocessing.connection to implement this. The following is a working example to demonstrate the concept:

import multiprocessing.connection
import sys

SERVER_HOST = 'localhost'
SERVER_PORT = 6000

g_total = 0


def process_numbers(numbers):
    global g_total
    for number in numbers:
        g_total += number
        print('Adding number %d. Total is now %d.' % (number, g_total))


def server_main(listener, numbers):
    process_numbers(numbers)
    print('Starting server.')
    while True:
        client = listener.accept()
        numbers = list(client.recv())
        client.close()
        if not numbers:
            break
        process_numbers(numbers)


def client_main(numbers):
    client = multiprocessing.connection.Client((SERVER_HOST, SERVER_PORT), 'AF_INET')
    print('Starting client.')
    client.send(numbers)


def main():
    numbers = map(int, sys.argv[1:])
    try:
        listener = multiprocessing.connection.Listener((SERVER_HOST, SERVER_PORT), 'AF_INET')
    except OSError:
        client_main(numbers)
    else:
        server_main(listener, numbers)

if __name__ == '__main__':
    main()

Assuming you have saved the above code as main.py, calling py main.py 1 2 3 will print the following output:

Adding number 1. Total is now 1.
Adding number 2. Total is now 3.
Adding number 3. Total is now 6.
Starting server.

The application will keep running. Now open a second terminal and run py main.py 4 5 6. The script will only print:

Starting client.

However, your first application (acting as the server) will now print:

Adding number 4. Total is now 10.
Adding number 5. Total is now 15.
Adding number 6. Total is now 21.

If you repeat the command it prints:

Adding number 4. Total is now 25.
Adding number 5. Total is now 30.
Adding number 6. Total is now 36.

and so on. You can quit the server by calling the client without arguments.

Answered By - wovano