Python Process Pool non-daemonic?

Would it be possible to create a python Pool that is non-daemonic? I want a pool to be able to call a function that has another pool inside.

I want this because deamon processes cannot create process. Specifically, it will cause the error:

AssertionError: daemonic processes are not allowed to have children

For example, consider the scenario where function_a has a pool which runs function_b which has a pool which runs function_c. This function chain will fail, because function_b is being run in a daemon process, and daemon processes cannot create processes.

The multiprocessing.pool.Pool class creates the worker processes in its __init__ method, makes them daemonic and starts them, and it is not possible to re-set their daemon attribute to False before they are started (and afterwards it's not allowed anymore). But you can create your own sub-class of multiprocesing.pool.Pool (multiprocessing.Pool is just a wrapper function) and substitute your own multiprocessing.Process sub-class, which is always non-daemonic, to be used for the worker processes.

Here's a full example of how to do this. The important parts are the two classes NoDaemonProcess and MyPool at the top and to call pool.close() and pool.join() on your MyPool instance at the end.

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

import multiprocessing
# We must import this explicitly, it is not imported by the top-level
# multiprocessing module.
import multiprocessing.pool
import time

from random import randint


class NoDaemonProcess(multiprocessing.Process):
    # make 'daemon' attribute always return False
    def _get_daemon(self):
        return False
    def _set_daemon(self, value):
        pass
    daemon = property(_get_daemon, _set_daemon)

# We sub-class multiprocessing.pool.Pool instead of multiprocessing.Pool
# because the latter is only a wrapper function, not a proper class.
class MyPool(multiprocessing.pool.Pool):
    Process = NoDaemonProcess

def sleepawhile(t):
    print("Sleeping %i seconds..." % t)
    time.sleep(t)
    return t

def work(num_procs):
    print("Creating %i (daemon) workers and jobs in child." % num_procs)
    pool = multiprocessing.Pool(num_procs)

    result = pool.map(sleepawhile,
        [randint(1, 5) for x in range(num_procs)])

    # The following is not really needed, since the (daemon) workers of the
    # child's pool are killed when the child is terminated, but it's good
    # practice to cleanup after ourselves anyway.
    pool.close()
    pool.join()
    return result

def test():
    print("Creating 5 (non-daemon) workers and jobs in main process.")
    pool = MyPool(5)

    result = pool.map(work, [randint(1, 5) for x in range(5)])

    pool.close()
    pool.join()
    print(result)

if __name__ == '__main__':
    test()

I had the necessity to employ a non-daemonic pool in Python 3.7 and ended up adapting the code posted in the accepted answer. Below there's the snippet that creates the non-daemonic pool:

import multiprocessing.pool

class NoDaemonProcess(multiprocessing.Process):
    @property
    def daemon(self):
        return False

    @daemon.setter
    def daemon(self, value):
        pass


class NoDaemonContext(type(multiprocessing.get_context())):
    Process = NoDaemonProcess

# We sub-class multiprocessing.pool.Pool instead of multiprocessing.Pool
# because the latter is only a wrapper function, not a proper class.
class NestablePool(multiprocessing.pool.Pool):
    def __init__(self, *args, **kwargs):
        kwargs['context'] = NoDaemonContext()
        super(NestablePool, self).__init__(*args, **kwargs)

As the current implementation of multiprocessing has been extensively refactored to be based on contexts, we need to provide a NoDaemonContext class that has our NoDaemonProcess as attribute. NestablePool will then use that context instead of the default one.

That said, I should warn that there are at least two caveats to this approach:

It still depends on implementation details of the multiprocessing package, and could therefore break at any time.
There are valid reasons why multiprocessing made it so hard to use non-daemonic processes, many of which are explained here. The most compelling in my opinion is:

As for allowing children threads to spawn off children of its own using subprocess runs the risk of creating a little army of zombie 'grandchildren' if either the parent or child threads terminate before the subprocess completes and returns.

Python Process Pool non-daemonic?

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