How can I stop a worker thread in tkinter when window is closed?

When I close the tkinter window while the worker thread is running I get this error after 10 seconds: RuntimeError: main thread is not in main loop probably because ROOT is destroyed.

import tkinter as tk
from tkinter import ttk
import threading
import time


def worker(event_ready):
    event_ready.wait()

    progress_bar = ttk.Progressbar(ROOT, orient='horizontal', mode='indeterminate', length=280)
    progress_bar.grid(column=0, row=0, columnspan=2, padx=10, pady=20)

    progress_bar.start()
    time.sleep(10)
    progress_bar.stop()



event_ready = threading.Event()
worker_thread = threading.Thread(target=worker, args=(event_ready,))
worker_thread.start()

global ROOT
ROOT = tk.Tk()
ROOT.geometry('300x120')
ROOT.title('TEST')
ROOT.grid()
event_ready.set()
ROOT.mainloop()

How can stop worker thread a soon as I close window?

You simply only need to start the Thread as a daemon, at least in Windows.

worker_thread = threading.Thread(target=worker, args=(event_ready,), daemon=True)

Now when you close the Window, the program will terminate immediately.

To stop the worker thread as soon as you close the tkinter window, you can use the protocol method in the Tk class to handle the window closing event. This allows you to define a function that will be called when the window is closed.

import tkinter as tk
from tkinter import ttk
import threading
import time

def worker(event_ready, stop_event):
    event_ready.wait()
    progress_bar = ttk.Progressbar(
        ROOT,
        orient='horizontal',
        mode='indeterminate',
        length=280
    )
    progress_bar.grid(column=0, row=0, columnspan=2, padx=10, pady=20)

    progress_bar.start()
    while not stop_event.is_set():
        time.sleep(0.1)
    progress_bar.stop()
    ROOT.destroy()


def on_closing():
    stop_event.set()


event_ready = threading.Event()
stop_event = threading.Event()
worker_thread = threading.Thread(target=worker, args=(event_ready, stop_event), daemon=True)
worker_thread.start()

ROOT = tk.Tk()
ROOT.geometry('300x120')
ROOT.title('TEST')
ROOT.grid()
event_ready.set()

ROOT.protocol("WM_DELETE_WINDOW", on_closing) 
ROOT.mainloop()

The on_closing function is bound to the window's closing event using ROOT.protocol, and it simply sets the stop_event flag, signaling the worker thread to stop.

This way, when you close the tkinter window, the on_closing function will be called, stopping the worker thread, and then the window will be closed gracefully.