Python threads using the same variables

Question

I have a while True loop that needs to be run in the background to update a variable and a function that needs to return that same variable. I tried this as a test:

import threading
downspeed = 0

def updating():
    while True:
        downspeed = downspeed+1

def main():
    while True:
        print(downspeed)


u = threading.Thread(name='background', target=updating)
m = threading.Thread(name='foreground', target=main)

u.start()
m.start()

But it only returns 0

if you want downspeed to be a global variable, you should put `global downspeed` at the beginning of the function definition of `updating` (inside the function). for `main`, since `downspeed` is not defined in the function scope, it will use the global scope, but if you set a variable within a function, it will become local unless you use `global` (and in this case `updating` should error, I believe) — hyper-neutrino, Jul 26 '21 at 19:57

score 1 · Accepted Answer · answered Jul 26 '21 at 20:00

Your code is not really a sufficient test. The test ought to include:

Allowing some time for the threads to run to see what they do.
Make the threads sleep, at least for a little time, to allow task switches to occur.

See this code:

import threading
import time

can_run = True
downspeed = 0

def updating():
    global downspeed
    while can_run:
        downspeed = downspeed+1
        time.sleep(0.1)   # Allow taskswitches

def main():
    while can_run:
        print(downspeed)
        time.sleep(1)   # Allow taskswitches


u = threading.Thread(name='background', target=updating)
m = threading.Thread(name='foreground', target=main)

u.start()
m.start()

time.sleep(10)  # Allow time for the threads to do what they want
can_run = False
print('Done')
u.join()
m.join()

There are no problems sharing variables in python because of the GIL. Although this only makes changes atomic.

Python threads using the same variables

1 Answers1