How to use Values in a multiprocessing pool with Python

I want to be able to use the Values module from the multiprocessing library to be able to keep track of data. As far as I know, when it comes to multiprocessing in Python, each process has it's own copy, so I can't edit global variables. I want to be able to use Values to solve this issue. Does anyone know how I can pass Values data into a pooled function?

from multiprocessing import Pool, Value
import itertools

arr = [2,6,8,7,4,2,5,6,2,4,7,8,5,2,7,4,2,5,6,2,4,7,8,5,2,9,3,2,0,1,5,7,2,8,9,3,2,]

def hello(g, data):
    data.value += 1

if __name__ == '__main__':
    data = Value('i', 0)
    func = partial(hello, data)
    p = Pool(processes=1)
    p.map(hello,itertools.izip(arr,itertools.repeat(data)))

    print data.value

Here is the runtime error i'm getting:

RuntimeError: Synchronized objects should only be shared between processes through inheritance

Does anyone know what I'm doing wrong?

Solution

I don't know why, but there seems to be some issue using the Pool that you don't get if creating subprocesses manually. E.g. The following works:

from multiprocessing import Process, Value

arr = [1,2,3,4,5,6,7,8,9]


def hello(data, g):
    with data.get_lock():
        data.value += 1
    print id(data), g, data.value

if __name__ == '__main__':
    data = Value('i')
    print id(data)

    processes =  []
    for n in arr:
        p = Process(target=hello, args=(data, n))
        processes.append(p)
        p.start()

    for p in processes:
        p.join()

    print "sub process tasks completed"
    print data.value

However, if you do basically the same think using Pool, then you get an error "RuntimeError: Synchronized objects should only be shared between processes through inheritance". I have seen that error when using a pool before, and never fully got to the bottom of it.

An alternative to using Value that seems to work with Pool is to use a Manager to give you a 'shared' list:

from multiprocessing import Pool, Manager
from functools import partial


arr = [1,2,3,4,5,6,7,8,9]


def hello(data, g):
    data[0] += 1


if __name__ == '__main__':
    m = Manager()
    data = m.list([0])
    hello_data = partial(hello, data)
    p = Pool(processes=5)
    p.map(hello_data, arr)

    print data[0]