How to observe a variable with a thread?

Question

Is it possible to observe the current value of a variable with a thread? Example: I have a variable which changes it's value every second and the function check(), which should print "over 10" as soon as the variable is over 10.

Any ideas ?

import threading
import time


def check (timer):

    print("thread started")
    while True:
        if timer > 10:
           print("over 10")


timer = 0
threading.Thread(target= check, name= "TimerThread", args=((timer,))).start()


while True:
    print(str(timer))
    timer = timer + 1
    time.sleep(1)

Solomon Slow · Accepted Answer · 2020-05-01 14:35:01Z

1

The timer in your check() function is not the same variable as the top-level timer variable. The one in check() is a local.

Try changing check() like this:

def check ():
    global timer
    ...the rest is unchanged...

The global keyword allows the check() function to see the top-level timer.

Then, since check() no longer expects an argument, you can start it more simply:

threading.Thread(target= check, name= "TimerThread").start()

answered May 1, 2020 at 14:35

Solomon Slow

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1 Comment

Alfred Wächter Over a year ago

Thank you very much. Everything is working as expected now.

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