How to gather results from multiprocesses?

Question

I have a problem with my python code. What I want is that each process writes in one dictionary. What I get is that every process writes into his own dictionary.

To make it clear: After running the code: I get this output:

P 0: {0: 1}
P 2: {2: 1}
P 4: {4: 1}
P 6: {6: 1}
P 8: {8: 1}
All: {}

What I want is:

P 0: {0: 1}
P 2: {2: 1}
P 4: {4: 1}
P 6: {6: 1}
P 8: {8: 1}
All: {0: 1, 2: 1, 4: 1, 6: 1, 8: 1}

Here is my sample Code:

from multiprocessing import Process, Lock, cpu_count

class multiprocessingExample():

    global d
    d = {}
    global lock
    lock = Lock()

    def __init__(self):
        pass

    def proc(self, num):

            global lock
            global d
            with lock:
                if(num in d):
                    d[num] = d[num] + 1
                else:
                    d[num] = 1
                print("P " + str(num) + ": " + str(d))

    def main(self):
        jobs = []

        for i in range(0, 10):
            if(i%2 == 0):
                p = Process(target=self.proc, args=(i,))
                jobs.append(p)

        for job in jobs:
            job.start()

        for job in jobs:
            job.join()

        print("All: " + str(d))

obj = multiprocessingExample()
obj.main()

It'd be great if you could tell me what's going wrong.

Answer 1

Don't use global, use a Manager.dict:

from multiprocessing import Process, Lock, Manager


class multiprocessingExample():
    def __init__(self):
        self.m = Manager()
        self.d = self.m.dict()
        self.lock = Lock()

    def proc(self, num):
        with self.lock:
            if (num in self.d):
                self.d[num] = d[num] + 1
            else:
                self.d[num] = 1
            print("P " + str(num) + ": " + str(self.d))   
    def main(self):
        jobs = []
            for i in range(0, 10):
            if (i % 2 == 0):
                p = Process(target=self.proc, args=(i,))
                jobs.append(p)    

        for job in jobs:
            job.start()    
        for job in jobs:
            job.join()    
        print("All: " + str(self.d))


obj = multiprocessingExample()
obj.main()

Which will output something like:

P 0: {0: 1}
P 2: {0: 1, 2: 1}
P 4: {0: 1, 2: 1, 4: 1}
P 8: {0: 1, 8: 1, 2: 1, 4: 1}
P 6: {0: 1, 8: 1, 2: 1, 4: 1, 6: 1}
All: {0: 1, 8: 1, 2: 1, 4: 1, 6: 1}