Killing child processes created in class __init__ in Python

Question

(New to Python and OO - I apologize in advance if I'm being stupid here)

I'm trying to define a Python 3 class such that when an instance is created two subprocesses are also created. These subprocesses do some work in the background (sending and listening for UDP packets). The subprocesses also need to communicate with each other and with the instance (updating instance attributes based on what is received from UDP, among other things).

I am creating my subprocesses with os.fork because I don't understand how to use the subprocess module to send multiple file descriptors to child processes - maybe this is part of my problem.

The problem I am running into is how to kill the child processes when the instance is destroyed. My understanding is I shouldn't use destructors in Python because stuff should get cleaned up and garbage collected automatically by Python. In any case, the following code leaves the children running after it exits.

What is the right approach here?

import os
from time import sleep

class A:
    def __init__(self):
        sfp, pts = os.pipe() # senderFromParent, parentToSender
        pfs, stp = os.pipe() # parentFromSender, senderToParent
        pfl, ltp = os.pipe() # parentFromListener, listenerToParent
        sfl, lts = os.pipe() # senderFromListener, listenerToSender
        pid = os.fork()
        if pid:
            # parent
            os.close(sfp)
            os.close(stp)
            os.close(lts)
            os.close(ltp)
            os.close(sfl)
            self.pts = os.fdopen(pts, 'w') # allow creator of A inst to
            self.pfs = os.fdopen(pfs, 'r') # send and receive messages
            self.pfl = os.fdopen(pfl, 'r') # to/from sender and
        else:                              # listener processes
            # sender or listener
            os.close(pts)
            os.close(pfs)
            os.close(pfl)
            pid = os.fork()
            if pid:
                # sender
                os.close(ltp)
                os.close(lts)
                sender(self, sfp, stp, sfl)
            else:
                # listener
                os.close(stp)
                os.close(sfp)
                os.close(sfl)
                listener(self, ltp, lts)

def sender(a, sfp, stp, sfl):
    sfp = os.fdopen(sfp, 'r') # receive messages from parent
    stp = os.fdopen(stp, 'w') # send messages to parent
    sfl = os.fdopen(sfl, 'r') # received messages from listener
    while True:
        # send UDP packets based on messages from parent and process
        # responses from listener (some responses passed back to parent)
        print("Sender alive")
        sleep(1)

def listener(a, ltp, lts):
    ltp = os.fdopen(ltp, 'w') # send messages to parent
    lts = os.fdopen(lts, 'w') # send messages to sender
    while True:
        # listen for and process incoming UDP packets, sending some
        # to sender and some to parent
        print("Listener alive")
        sleep(1)

a = A()

Running the above produces:

Sender alive
Listener alive
Sender alive
Listener alive
...

Answer 1

Actually, you should use destructors. Python objects have a __del__ method, which is called just before the object is garbage-collected.

In your case, you should define

def __del__(self):
   ...

within your class A that sends the appropriate kill signals to your child processes. Don't forget to store the child PIDs in your parent process, of course.