Python - tkinter - root.after (reddit.com)

import tkinter as tk

# --- constants --- (UPPER_CASE)

START = 1
STOP  = 0
EXIT  = -1

# --- functions ---

def test_loop():

    if looping == START:
        print("test")
    if looping == EXIT:
        return

    # run again after 1000ms = 1s
    root.after(1000, test_loop)


def start():
    global looping

    looping = START


def stop():
    global looping

    looping = STOP

# --- main ---

looping = STOP

root = tk.Tk()

text = tk.StringVar()
text.set("Here's some text!")

leftFrame = tk.Frame(root)
leftFrame.pack(side=tk.LEFT)

rightFrame = tk.Frame(root)
rightFrame.pack(side=tk.RIGHT)

playButton = tk.Button(leftFrame, text="Play", fg="blue", command=start)
playButton.pack(side=tk.TOP)

stopButton = tk.Button(rightFrame, text="Stop", fg="red", command=stop)
stopButton.pack(side=tk.BOTTOM)

message = tk.Message(root, textvariable=text, relief=tk.RAISED)
message.pack()

# start endless loop
test_loop()

root.mainloop()

looping = EXIT