Class Variables and Threading- An unexpected behavior

Question

I've encountered an unexpected behavior with threading. I've attached a short example below to demonstrate the behavior:

from multiprocessing import Process, Lock
import time

class inc:

    def __init__(self):
        print "Initializing a new class"
        self.value = 0

    def add_one(self):
        self.value += 1
        print self.value

def f(a,l):
    # Do something using a class call
    l.acquire()
    a.add_one()
    l.release()
    #Do something that takes a long time
    time.sleep(5)
    #Do a different thing using a class call
    l.acquire()
    a.add_one()
    l.release()

if __name__=="__main__":
    a=inc()
    lock = Lock()

    for i in range(0,4):
        Process(target=f, args=(a,lock)).start()

The output I'm looking for is 1,2,3,4,5,6,7,8 but I get: 1,1,1,1,2,2,2,2. Am I using thread locking properly? Only one instance of the class exists and I serialize thread access to the class, so why doesn't the class variable increment?

Roman Bodnarchuk · Accepted Answer · 2011-08-27 07:27:43Z

3

You are not using "thread locking", you are using process locking. Since processes don't share heap content, each time f gets different object.

See threading - Python library for operations with threads. It allows you to achieve needed behavior.

Probably just replace

from multiprocessing import Process, Lock
...
    Process(target=f, args=(a,lock)).start()

with

from threading import Thread, Lock
...
    Thread(target=f, args=(a,lock)).start()

edited Aug 27, 2011 at 7:27

answered Aug 27, 2011 at 7:20

Roman Bodnarchuk

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1 Comment

Chris Over a year ago

Thanks, that works! In hindsight, the answer to my question was fairly simple, but there's a huge amount of complexity to those simple changes. I'll be spending some extra time looking at the differences between threading and multiprocessing. Thanks for the quick, accurate and helpful answer.

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