Is it possible to spawn a process inside a thread in Python?

Question

I'm wrting a program that spawns a process and restarts the process on certain conditions. For example, if a child process doesn't send data anymore to the mother process, for a certain period of time, I want the mother process to terminate the child process and restart it. I thought I could use a thread to recieve data from a child process and restart the child process, but it doesn't work the way I thought.

import numpy as np
import multiprocessing as mp
import threading
import time
from apscheduler.schedulers.background import BackgroundScheduler

pipe_in, pipe_out = mp.Pipe()

class Mother():
    def __init__(self):
        self.pipe_out = pipe_out

        self.proc = mp.Process(target = self.test_func, args=(pipe_in, ))
        self.proc.start()

        self.thread = threading.Thread(target=self.thread_reciever, args=(self.pipe_out, ))
        self.thread.start()

    def thread_reciever(self, pipe_out):
        while True:
            value = pipe_out.recv()

            print(value)
            if value == 5:
                self.proc.terminate()
                time.sleep(2)
                self.proc = mp.Process(target = self.test_func)
                self.proc.start()

    def test_func(self, pipe_in):
        for i in range(10):
            pipe_in.send(i)
            time.sleep(1)


if __name__ == '__main__':
    r = Mother()

It prints out this error.

D:\>d:\python36-32\python.exe temp06.py
    0
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5
Exception in thread Thread-1:
Traceback (most recent call last):
File "d:\python36-32\lib\threading.py", line 916, in _bootstrap_inner
    self.run()
File "d:\python36-32\lib\threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
File "temp06.py", line 28, in thread_reciever
    self.proc.start()
File "d:\python36-32\lib\multiprocessing\process.py", line 105, in start
    self._popen = self._Popen(self)
File "d:\python36-32\lib\multiprocessing\context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
File "d:\python36-32\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
File "d:\python36-32\lib\multiprocessing\popen_spawn_win32.py", line 65, in __init__
    reduction.dump(process_obj, to_child)
File "d:\python36-32\lib\multiprocessing\reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: can't pickle _thread.lock objects


D:\>Traceback (most recent call last):
File "<string>", line 1, in <module>
File "d:\python36-32\lib\multiprocessing\spawn.py", line 99, in spawn_main
    new_handle = reduction.steal_handle(parent_pid, pipe_handle)
File "d:\python36-32\lib\multiprocessing\reduction.py", line 82, in steal_handle
    _winapi.PROCESS_DUP_HANDLE, False, source_pid)
OSError: [WinError 87]

How could I start and terminate a process inside a thread? (I'm using a thread because it can synchronously recieve data from a different process) Or are there any other ways to do this job?

test_func as a global function

import numpy as np
import multiprocessing as mp
import threading
import time
from apscheduler.schedulers.background import BackgroundScheduler

pipe_in, pipe_out = mp.Pipe()  

def test_func( pipe_in):
    for i in range(10):
        pipe_in.send(i)
        time.sleep(1)

class Mother():
    def __init__(self):
        self.pipe_out = pipe_out
        mp.freeze_support()
        self.proc = mp.Process(target = test_func, args=(pipe_in, ))
        self.proc.start()

        self.thread = threading.Thread(target=self.thread_reciever, args=(self.pipe_out, ))
        self.thread.start()

    def thread_reciever(self, pipe_out):
        while True:
            value = pipe_out.recv()

            print(value)
            if value == 5:
                self.proc.terminate()
                time.sleep(2)
                mp.freeze_support()
                self.proc = mp.Process(target = test_func, args=(pipe_in,))
                self.proc.start()


if __name__ == '__main__':

    r = Mother()

OUTPUT

D:\> d:\python36-32\python.exe temp06.py
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2
3
4
5
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "d:\python36-32\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
File "d:\python36-32\lib\multiprocessing\spawn.py", line 115, in _main
    self = reduction.pickle.load(from_parent)
AttributeError: Can't get attribute 'test_func' on <module '__main__' (built-in)>

Answer 1

under windows, as there is no fork syscall, python starts a new interpreter instance, use pickle/unpickle to reconstruct execution context, but thread.Lock is not picklable. while pickling self.test_func, self.thread reference to a thread.Lock object, makes it unpicklable.

you could simply change test_func to a plain global function, without thread object reference :

self.proc = mp.Process(target = test_func, args=(pipe_in,))
...
def test_func(pipe_in):
    for i in range(10):
        pipe_in.send(i)
        time.sleep(1)