How to start and stop multiple child processes from a class?

Question

The Python program:

import multiprocessing
import time


class Application:

    def __init__(self):
        self._event = multiprocessing.Event()
        self._processes = [
            multiprocessing.Process(target=self._worker)
            for _ in range(multiprocessing.cpu_count())]

    def _worker(self):
        while not self._event.is_set():
            print(multiprocessing.current_process().name)
            time.sleep(1)

    def start(self):
        for process in self._processes:
            print('starting')
            process.start()

    def stop(self):
        self._event.set()
        for process in self._processes:
            process.join()


if __name__ == '__main__':
    application = Application()
    application.start()
    time.sleep(3)
    application.stop()

Its output:

starting
starting
Traceback (most recent call last):
  File "/Users/maggyero/Desktop/application.py", line 31, in <module>
    application.start()
  File "/Users/maggyero/Desktop/application.py", line 21, in start
    process.start()
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'weakref' object
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
  File "/usr/local/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/synchronize.py", line 110, in __setstate__
    self._semlock = _multiprocessing.SemLock._rebuild(*state)
FileNotFoundError: [Errno 2] No such file or directory

In the function Application.__init__, each call multiprocessing.Process(target=self._worker) initializes a multiprocessing.Process instance with the instance method self._worker as its target argument. self._worker is bound to self which has the instance attribute self._processes.

In the function Application.start, each call process.start() serialises the target argument and therefore self._processes. self._processes is a list of multiprocessing.Process instances, initially not started yet. The first call process.start() starts the first multiprocessing.Process instance in that list without issue, but the second call process.start() fails.

So a started multiprocessing.Process instance cannot be serialised. How to solve that problem?