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terminate with extreme prejudice

Capturing the protocol event isn’t enough

Your code captures the protocol event and redefines its behavior. But, what
about the behavior that used to execute by default?
Take a look at this small example program, which redefines the close-box
protocol to check with the user before actually destroying the window:

from tkinter import *
from tkinter.messagebox import askokcancel

app = Tk()
app.title("Capturing Events")
app.geometry('250x100+200+200')

def shutdown():
if askokcancel(title = 'Are you sure?', message = 'Do you really want to quit?'):
app.destroy()

app.protocol("WM_DELETE_WINDOW", shutdown)
app.mainloop()

Clicking on the close box runs the
“shutdown()” function, which then
displays an AskOkCancel dialog.
Do this!
If you click OK, the GUI is
destroyed (which has the effect of
closing the window).

Add the app.destroy() line of code to the end of your
shutdown() function and see if it makes any difference.

326 Chapter 9