Question

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 the 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 {}

Answer 1

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))