[Tkinter] Tk window not displaying until loop has completed

[deanhystad]

tkinter updates things when your program is idle. Normally this is what you want to happen. If you have something that takes a long time to complete you should break it up into smaller pieces of processing, or run it in a thread or parallel process. This way your GUI is still responsive to user actions.

You will not see anything before mainloop() is called. Here is a much longer delay with a lot less going on.

import tkinter as tk
import time

start = time.time()
root = tk.Tk()
tk.Label(root, text='Hello World!').pack()
print(time.time() - start)
time.sleep(5)
root.mainloop()

If you don't care about having a responsive interface, you can call update() to update your widgets.

import tkinter as tk
import time

start = time.time()
root = tk.Tk()
tk.Label(root, text='Hello World!').pack()
root.update()
print(time.time() - start)
time.sleep(5)
root.mainloop()

This draws the window right away, but the window is unresponsive and you get a busy cursor until the program wakes up from its 5 second nap.