Passing variable changes between threads in Python functions [Beginner]

Question

So I have this code:

import time
import threading

bar = False

def foo():
    while True:
        if bar == True:
            print "Success!"
        else:
            print "Not yet!"
    time.sleep(1)

def example():
    while True:
        time.sleep(5)
        bar = True

t1 = threading.Thread(target=foo)
t1.start()

t2 = threading.Thread(target=example)
t2.start()

I'm trying to understand why I can't get bar to = to true.. If so, then the other thread should see the change and write Success!

The bar in the two functions are not in the same scope. You should deal with scopes before you learn multithreading. In any case there should be mutual-resource constructs you can use for threads. — Waleed Khan
– Waleed Khan, Commented Mar 6, 2013 at 17:58
Indentation in time.sleep(1) is wrong. I think is was intended to be inside the while loop. — Diego Herranz
– Diego Herranz, Commented Mar 6, 2013 at 18:21

Diego Herranz · Accepted Answer · 2013-03-06 18:12:19Z

11

bar is a global variable. You should put global bar inside example():

def example():
    global bar
    while True:
        time.sleep(5)
        bar = True

When reading a variable, it is first searched inside the function and if not found, outside. That's why it's not necessary to put global bar inside foo().
When a variable is assigned a value, it is done locally inside the function unless the global statement has been used. That's why it's necessary to put global bar inside example()

edited Mar 6, 2013 at 18:12

answered Mar 6, 2013 at 17:58

Diego Herranz

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Comments

user2665694 · Accepted Answer · 2013-03-06 17:58:35Z

1

You must specify 'bar' as global variable. Otherwise 'bar' is only considered as a local variable.

def example():
    global bar
    while True:
        time.sleep(5)
        bar = True

answered Mar 6, 2013 at 17:58

user2665694

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