Need help in updating a global variable by a thread

Bruno Desthuilliers bdesth.quelquechose at
Wed Oct 17 22:57:10 CEST 2007

dedalusenator at a écrit :
> Hello Folks,
> My first posting here and I am a stuck in figuring out the exact way
> to update a global variable from within a function that doesnt return
> any value (because the function is a target of the thread and I dont
> know how exactly return would work in such a case). I am sure I am
> missing something very fundamental here. The essential pieces of my
> code that cause the problem would be something like this:
> ---------------------------------------------
> lookuptab = {'':'Bangkok','':'Sydney'}
> results = {}
> for val in lookuptab.values():
>     results[val]=0
> def testt(loc):
>        global results
>        results[loc] = 1
>        return results[loc]
> for x in lookuptab.values():
>       thread = threading.Thread(target=testt,args=(x))
>       thread.start()
> print results
> -------------------------------------------------------

"Would be" ?

I had to fix a couple problems to get your code running (namely, 
importing threading and passing correct args to threading.Thread). Do 
yourself a favour: next time, take time to post *working* code.

Anyway... Here's a (corrected) version with a couple prints here and 
there. I think the output is clear enough:

import threading
import time

lookuptab = {'':'Bangkok','':'Sydney'}
results = dict((val, 0) for val in lookuptab.values())

def testt(loc):
        global results
        print "t-%s before: %s" % (loc,results)
        results[loc] = 1
        print "t-%s after: %s" % (loc,results)

def main():
       for x in lookuptab.values():
             thread = threading.Thread(target=testt,args=(x,))

       print "main - no sleep: %s" % results
       print "main - 1s later : %s" % results

if __name__ == '__main__': main()

And the output is:

main - no sleep: {'Bangkok': 0, 'Sydney': 0}
t-Bangkok before: {'Bangkok': 0, 'Sydney': 0}
t-Bangkok after: {'Bangkok': 1, 'Sydney': 0}
t-Sydney before: {'Bangkok': 1, 'Sydney': 0}
t-Sydney after: {'Bangkok': 1, 'Sydney': 1}
main - 1s later : {'Bangkok': 1, 'Sydney': 1}

Now if I may give you an advice about threads and globals (or any other 
kind of shared state): learn about semaphores. While this may not be an 
issue in this snippet, race conditions is definitively something you 
want to avoid whenever possible and cleanly handle else.


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