Using multiprocessing to produce objects for i in range

[deanhystad]

This should work on linux, but it will not work on windows. In linux a process is forked, starting out with the same process image as the parent. In windows a process is spawned, starting out as a completely new process.

To have a variable that is the same for all processes, you need to pass it as an argument to the process.

From the docs: https://docs.python.org/3/library/multiprocessing.html

from multiprocessing import Process, Value, Array

#Data can be stored in a shared memory map using Value or Array. For example, the following code

def f(n, a):
    n.value = 3.1415927
    for i in range(len(a)):
        a[i] = -a[i]

if __name__ == '__main__':
    num = Value('d', 0.0)
    arr = Array('i', range(10))

    p = Process(target=f, args=(num, arr))
    p.start()
    p.join()

    print(num.value)
    print(arr[:])

Or you can use a process manager. From the same document.

from multiprocessing import Process, Manager

def f(d, l):
    d[1] = '1'
    d['2'] = 2
    d[0.25] = None
    l.reverse()

if __name__ == '__main__':
    with Manager() as manager:
        d = manager.dict()
        l = manager.list(range(10))

        p = Process(target=f, args=(d, l))
        p.start()
        p.join()

        print(d)
        print(l)