python mulitprocessing.Queue test for each Context

mp_queue_test.py

import multiprocessing
import argparse

queue = None


def task(queue, value):
    queue.put(value)


def task2(value):
    queue.put(value)


'''
$ python mp_queue_test.py --start_method fork
1 2 3

$ python mp_queue_test.py --start_method spawn
Process Process-3:
Traceback (most recent call last):
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.7.2/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.7.2/lib/python3.7/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/kazuhiroserizawa/Documents/python-work/experiments/mp_queue_test.py", line 12, in task2
    queue.put(value)
AttributeError: 'NoneType' object has no attribute 'put'

$ python mp_queue_test.py --start_method forkserver
Process Process-3:
Traceback (most recent call last):
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.7.2/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.7.2/lib/python3.7/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/kazuhiroserizawa/Documents/python-work/experiments/mp_queue_test.py", line 12, in task2
    queue.put(value)
AttributeError: 'NoneType' object has no attribute 'put'
'''
def main():
    global queue
    queue = multiprocessing.Queue(100)
    p1 = multiprocessing.Process(target=task, args=[queue, 1])
    p2 = multiprocessing.Process(target=task, args=[queue, 2])
    p3 = multiprocessing.Process(target=task2, args=[3])
    p1.start()
    p2.start()
    p3.start()
    p1.join()
    p2.join()
    p3.join()
    print(queue.get(), queue.get(), queue.get())


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--start_method", type=str, default="fork")
    args = parser.parse_args()

    multiprocessing.set_start_method(args.start_method)

    main()