Python: how to share a variable between two modules

Question

I have viewed some other threads that address the problem but not quite get it.

Suppose I have a config file that contains a shared variable:

Config.py

flag = False

Test1.py

import config
while(1):
    config.flag = True
    print config.flag

Test2.py

import config
while(1):
    config.flag = False
    print config.flag

If now I run Test1.py and Test2.py, can I see the switch between 'True' 'False' in the output? I don't care about synchronization at this point, as long as it can access the shared variable, then it's done.

Thanks.

Answer 1

No. As long as you're running both instances in separate processes, memory won't be magically shared.

You're starting two different (Python) processes. Those processes import the config module. Simplified, this is just a form of loading the code in config.py. There is no further communication going between the processes.

(As a side note, the code in config.py is only interpreted the first time, it's compiled and saved in a separate config.pyc which loads much faster. The next time config.py is edited, config.pyc will be recreated).

There are other options:

• Use threads
• Use some other form of memory sharing, for example pipes.

Example with threads:

config.py

flag = False

test.py

import thread
import time

import config


def test1():
    while 1:
        config.flag = True
        time.sleep(0.7)
        print 'test1:', config.flag
        time.sleep(1)

def test2():
    while 1:
        config.flag = False
        time.sleep(1.1)
        print 'test2:', config.flag
        time.sleep(1)

thread.start_new(test1, ())
test2()

It may be helpful to post the reason you're trying to do this. Multithreading is a difficult topic. This question may be useful: https://stackoverflow.com/questions/4690518/multithreading-in-python