Python Process won't call atexit

Question

I'm trying to use atexit in a Process, but unfortunately it doesn't seem to work. Here's some example code:

import time
import atexit
import logging
import multiprocessing

logging.basicConfig(level=logging.DEBUG)

class W(multiprocessing.Process):
    def run(self):
        logging.debug("%s Started" % self.name)

        @atexit.register
        def log_terminate():
             # ever called?
             logging.debug("%s Terminated!" % self.name)

        while True:
            time.sleep(10)

@atexit.register
def log_exit():
    logging.debug("Main process terminated")

logging.debug("Main process started")

a = W()
b = W()
a.start()
b.start()
time.sleep(1)
a.terminate()
b.terminate()

The output of this code is:

DEBUG:root:Main process started
DEBUG:root:W-1 Started
DEBUG:root:W-2 Started
DEBUG:root:Main process terminated

I would expect that the W.run.log_terminate() would be called when a.terminate() and b.terminate() are called, and the output to be something likeso (emphasis added)!:

DEBUG:root:Main process started
DEBUG:root:W-1 Started
DEBUG:root:W-2 Started
DEBUG:root:W-1 Terminated!
DEBUG:root:W-2 Terminated!
DEBUG:root:Main process terminated

Why isn't this working, and is there a better way to log a message (from the Process context) when a Process is terminated?

Thank you for your input - it's much appreciated.

Solution

EDIT: Based on solution suggested by Alex Martelli, the following works as expected:

import sys
import time
import atexit
import signal
import logging
import multiprocessing

logging.basicConfig(level=logging.DEBUG)

class W(multiprocessing.Process):
    def run(self):
        logging.debug("%s Started" % self.name)

        def log_terminate(num, frame):
             logging.debug("%s Terminated" % self.name)
             sys.exit()
        signal.signal(signal.SIGTERM, log_terminate)
        while True:
            time.sleep(10)

@atexit.register
def log_exit():
    logging.debug("Main process terminated")

logging.debug("Main process started")
a = W()
b = W()
a.start()
b.start()
time.sleep(1)
a.terminate()
b.terminate()

It's worthwhile to note the following comment in the atexit documentation:

Note: the functions registered via this module are not called when the program is killed by a signal, when a Python fatal internal error is detected, or when os._exit() is called.

Panegyrize

A possible workaround is to make use of undocumented multiprocessing.util.Finalize class, see the blog post for details: Guaranteed Finalization Without Context Manager. – Recommit

Answer 1

As the docs say,

On Unix this is done using the SIGTERM signal; on Windows TerminateProcess() is used. Note that exit handlers and finally clauses, etc., will not be executed.

If you're on Unix, you should be able intercept SIGTERM with signal, and perform whatever "termination activities" you need; however, I don't know of a cross-platform solution.

Holladay