Is it necessary to call Tk() when writing a GUI app with Tkinter?

Terry Reedy tjreedy at udel.edu
Thu Mar 1 01:17:09 CET 2012


On 2/29/2012 9:24 AM, Rick Johnson wrote:
> On Feb 28, 11:06 pm, John Salerno<johnj... at gmail.com>  wrote:

>> However, in the Python documentation, I see this:
>>
>> root = Tk()
>> app = Application(master=root)
>> app.mainloop()
>> root.destroy()

>> I tried the above and I got the following error:
>>
>> Traceback (most recent call last):
>>    File "C:\Users\John\Desktop\gui.py", line 12, in<module>
>>      root.destroy()
>>    File "C:\Python32\lib\tkinter\__init__.py", line 1714, in destroy
>>      self.tk.call('destroy', self._w)
>> _tkinter.TclError: can't invoke "destroy" command:  application has been destroyed
>>
>> So apparently closing the window with the X button (on Windows)
 >> implicitly calls the destroy() method of the root frame.
 >> If that's the case, why does the documentation explicitly call it?

I do not know if tk has changed since the example was written or if it 
was buggy from the beginning. I opened an issue to fix it.

http://bugs.python.org/issue14163

> Most applications will have both: user destroying, and program
> destroying.

> from tkMessageBox import askyesnocancel

from tkinter.messagebox in 3.x

> class App(tk.Tk):
>      def __init__(self):
>          tk.Tk.__init__(self)
>          self.title('Close Me -->')
>          self.protocol("WM_DELETE_WINDOW", self.onDestroyWindow)
>
>      def onDestroyWindow(self):
>          title = 'Confirm App Exit'
>          msg = 'Save changes before exiting?'
>          result = askyesnocancel(title, msg, default='cancel')
>          if result is None:
>              return
>          elif result is True:
>              print 'saving changes'
>          elif result is False:
>              print 'dont save changes'
>          self.destroy()
>
> if __name__ == '__main__':
>      app = App()
>      app.mainloop()

This works as adjusted for 3.x. I presume that a quit button or menu 
entry should also call onDestroyWindow so the effect is the same as 
clicking the outer [X] button.

I tried the same approach to fix the doc example, but unlike your class 
App(Tk), class App(Frame) does not a .protocol attribute. See the 
tracker issue for all my comments on the example.

I considered removing both the quit button and 'root.destroy' to get a 
beginning example that works properly, but as you said, having both is 
common so I would like both if the solution is not too esoteric.

-- 
Terry Jan Reedy




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