Question

I’m trying to use the multiprocessing module in python 2.6, but apparently there is something I do not understand. I would expect the class below to add up the numbers sent to it by add() and return the sum in the get_result() method. The code below prints “0”, I’d like it to print “2”. What have I missed?

import multiprocessing

class AdderProcess(multiprocessing.Process):

    def __init__(self):
        multiprocessing.Process.__init__(self)
        self.sum = 0
        self.queue = multiprocessing.JoinableQueue(5)
        self.daemon = True
        self.start()

    def run(self):
        while True:
            number = self.queue.get()
            self.sum += number
            self.queue.task_done()

    def add(self, number):
        self.queue.put(number)

    def get_result(self):
        self.queue.join()
        return self.sum


p = AdderProcess()
p.add(1)
p.add(1)
print p.get_result()

PS. This problem has been solved. Thanks for the answers! Just to make it easier for any readers, here’s the complete working version:

import multiprocessing

class AdderProcess(multiprocessing.Process):

    def __init__(self):
        multiprocessing.Process.__init__(self)
        self.sum = multiprocessing.Value('d', 0.0)
        self.queue = multiprocessing.JoinableQueue(5)
        self.daemon = True
        self.start()

    def run(self):
        while True:
            number = self.queue.get()
            self.sum.value += number
            self.queue.task_done()

    def add(self, number):
        self.queue.put(number)

    def get_result(self):
        self.queue.join()
        return self.sum.value

p = AdderProcess()
p.add(1)
p.add(1)
print p.get_result()

Answer 1

Change self.sum = 0 to self.sum = multiprocessing.Value('d', 0.0), and use self.sum.value to access or change the value.

class AdderProcess(multiprocessing.Process):    
    def __init__(self):
        ...
        self.sum = multiprocessing.Value('d', 0.0) 
        ...
    def run(self):
        while True:
            number = self.queue.get()
            self.sum.value += number    # <-- use self.sum.value
            self.queue.task_done()
    def get_result(self):
        self.queue.join()
        return self.sum.value           # <-- use self.sum.value

The problem is this: Once you call self.start() in __init__, the main process forks off a child process. All values are copied. Now there are two versions of p. In the main process, p.sum is 0. In the child process, the run method is called and p.sum is augmented to 2. But when the main process calls p.get_result(), its version of p still has p.sum equal to 0.
So 0 is printed.

When you want to share a float value between processes, you need to use a sharing mechanism, such as mp.Value.

See “Sharing state between processes” for more options on how to share values.