Global Variable not working with Threads - Python

Question

I want to be able to change the background colour of a Tkinter frame in a thread, the frame is declared in a separate function. I receive the following error when I run the following code.

Error: NameError: name 'mainScreen' is not defined

Code:

import tkinter as tk
from tkinter import ttk
from multiprocessing import Process


def main():
    global mainScreen
    
    root = tk.Tk()
    root.geometry('1040x540+50+50')

    mainScreen = tk.Frame(root, width = 1040, height = 540)
    mainScreen.place(x=0, y=0)

    root.mainloop()


def test(): # This function is in a thread as it will be run as a loop.
    while True:
        mainScreen.configure(bg='red')

if __name__ == '__main__':
    p2 = Process(target = test)
    p2.start()
    main()

Any help is appreciated.

You're starting the thread before you create the screen. Have you tried creating the screen before starting the thread? — Bryan Oakley
– Bryan Oakley, Commented Sep 25, 2020 at 17:17
If all you're doing is changing the color of a widget, threads are not at all necessary. — Bryan Oakley
– Bryan Oakley, Commented Sep 25, 2020 at 17:18

wind · Accepted Answer · 2020-09-25 10:10:22Z

1

You can replace your whole code with this:

import tkinter as tk

def main():
    global mainScreen

    root = tk.Tk()
    root.geometry('1040x540+50+50')

    mainScreen = tk.Frame(root, width=1040, height=540)
    mainScreen.place(x=0, y=0)
    mainScreen.configure(bg='red')
    root.mainloop()


if __name__ == '__main__':
    main()

And if you want to change colours you can do something like this:

import time
import tkinter as tk
from threading import Thread


def test(mainScreen):  # This function is in a thread as it will be run as a loop.
    while True:
        try:
            time.sleep(1)
            mainScreen.configure(bg='red')
            time.sleep(1)
            mainScreen.configure(bg='blue')
        except RuntimeError:
            break


if __name__ == '__main__':
    root = tk.Tk()
    root.geometry('1040x540+50+50')

    mainScreen = tk.Frame(root, width=1040, height=540)
    mainScreen.place(x=0, y=0)
    p2 = Thread(target=test, args=(mainScreen,))
    p2.start()

    root.mainloop()

edited Sep 25, 2020 at 10:10

answered Sep 25, 2020 at 9:33

wind

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4 Comments

ahmedquran12 Over a year ago

I know, but I'm making a loop to change the colour of the background.

wind Over a year ago

@ultimateduc Edited answer so you can use the while loop as well

ahmedquran12 Over a year ago

Doesn't work as I cant assign colours in the test function.

wind Over a year ago

@ultimateduc Fixed the problem

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