I've written the following code to illustrate the problem I'm seeing. I'm trying to use a Process.Manager.list() to keep track of a list and increment random indices of that list.

Each time there are 100 processes spawned, and each process increments a random index of the list by 1. Therefore, one would expect the SUM of the resulting list to be the same each time, correct? I get something between 203 and 205.

from multiprocessing import Process, Manager
import random

class MyProc(Process):
    def __init__(self, A):
        Process.__init__(self)
        self.A = A

    def run(self):
        i = random.randint(0, len(self.A)-1)
        self.A[i] = self.A[i] + 1

if __name__ == '__main__':
    procs = []
    M = Manager()
    a = M.list(range(15))
    print('A: {0}'.format(a))
    print('sum(A) = {0}'.format(sum(a)))

    for i in range(100):
        procs.append(MyProc(a))

    map(lambda x: x.start(), procs)
    map(lambda x: x.join(), procs)
    print('A: {0}'.format(a))
    print('sum(A) = {0}'.format(sum(a)))