Pass-by-reference mutable variable in a thread

Question

I have a basic question about multi-threading in Python: I have a list and I need modify this in a thread. I understand that an list is a mutable type: How do I pass a variable by reference?

But, when I using thread the list is not behaving like a mutable type:

from multiprocessing import Process, Lock

def f(l, i, n):
    l.acquire()
    i.append(n)
    l.release()
    print "in:", i

if __name__ == '__main__':
    lock = Lock()

    i = []
    for num in range(10):
        p = Process(target=f, args=(lock, i, num))
        p.start()
        p.join()

    print "out:", i

output

in: [0]
in: [1]
in: [2]
in: [3]
in: [4]
in: [5]
in: [6]
in: [7]
in: [8]
in: [9]
out: []

Can someone please help me with this issue?

Answer 1

The code is not using threads, but processes which does not share memory.

Use threads:

from threading import Thread, Lock  # <----

def f(l, i, n):
    l.acquire()
    i.append(n)
    l.release()
    print "in:", i

if __name__ == '__main__':
    lock = Lock()

    i = []
    for num in range(10):
        p = Thread(target=f, args=(lock, i, num))  # <----
        p.start()
        p.join()

    print "out:", i