Python threads and global vars

Question

Say I have the following function in a module called "firstModule.py":

def calculate(): # addCount value here should be used from the mainModule a=random.randint(0,5) + addCount

Now I have a different module called "secondModule.py":

def calculate(): # addCount value here too should be used from the mainModule a=random.randint(10,20) + addCount

I am running a module called "mainModule.py" which has the following (notice the global "addCount" var):

import firstModule import secondModule addCount=0 Class MyThread(Thread): def __init__(self,name): Thread.__init__(self) self.name=name def run(self): global addCount if self.name=="firstModule": firstModule.calculate() if self.name=="secondModule": secondModule.calculate() def main(): the1=MyThread("firstModule"); the2=MyThread("secondModule"); the1.start() the2.start() the1.join() the2.join() # This part doesn't work: print firstModule.a print secondModule.a

Basically I want the "addCount" value in both modules to be the one from "mainModule". After that, when the threads are finished, I want to print the value of "a" in both of them. The example above doesn't work. I was wondering how can I fix it.

suzanshakya · Accepted Answer · 2010-10-09 17:46:21Z

Pass 'addCount' to the function 'calculate', return the value of 'a' in 'calculate', and assign it to a new attribute in MyThread instance.

def calculate(addCount): a = random.randint(0, 5) + addCount return a

Lie Ryan · Accepted Answer · 2010-10-09 15:52:48Z

Modules in python are singletons, so you can put your global variables in module globalModule.py and have both firstModule, secondModule, and mainModule import globalModule and they will all access the same addCount.

However, in general it's a bad practice for threads to have a global state.

This will never work:

print firstModule.a print secondModule.a

because in here:

def calculate(): # addCount value here should be used from the mainModule a=random.randint(0,5) + addCount

a is a local variable to the function calculate.

If you really want to write a as a module-level variable, add global declaration:

def calculate(): # addCount value here should be used from the mainModule global a a=random.randint(0,5) + addCount

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