Python passing dictionary to process in multiprocessing

I have a class that contains a (large) number of different properties, including a few dictionaries. When I pass an instance of the class through to a new process, all of the numeric values seem to get passed in correctly, but any dictionaries that were in the class object get emptied.

Here's a simple test I cooked up that demonstrates my problem:

from multiprocessing import Process

class State:
    a = 0
    b = {}

def f(s, i):
    print "f:", s.a, s.b

def main():

    state = State()

    state.a = 11
    state.b['testing'] = 12

    print "Main:", state.a, state.b

    ps = []
    for i in range(1):
        p = Process(target=f, args=(state, i))
        p.start()           # Do th开发者_运维知识库e work
        ps.append(p)
    for p in ps:
        p.join()

if __name__ == '__main__':
    main()

I expect the output to be

Main: 11 {'testing': 12}
f: 11 {'testing': 12}

but instead I get

Main: 11 {'testing': 12}
f: 11 {}

the problem is clearly described in the documentation for the multiprocessing module: windows lacks a fork() system call, so on windows, the module source code gets reevaluated at the start of each process, thus the current state is not preserved. (on un*x based system, including OSX, Process is implemented using a fork, and the state is preserved)

note that i am talking about the state of the whole python module.

your way of holding a state is wrong: the a and b members of the State class are class-wide 'static' members, they are not part of an instance of a State object. when doing state.a = 11, you are modifying the State class, not an instance of the class.

your code would have worked fine if a and b had been instance members of the State class:

class State(object):
    def __init__(self)
        self.a = 0
        self.b = {}

then, passing the state instance to a new Process, would have correctly passed the value as expected.

I ended up working around this problem by passing the dictionaries in explicitly in addition to the state. e.g.

p = Process(target=f, args=(state, i, state.b))