Which tkinter attribute indicates if tkinter is still running?

Question

Is there a definitive attribute of Tk that I can check to see if either the mainloop has stopped running or if the root window has been destroyed?

The minimal code below shows a problem resulting from Tk’s apparent failure to propagate python exceptions. To see the problem in action, click on the root window button, “Start The Child Window Dialog”. Next, close the root window using its close window button (Red X).

import sys
import tkinter as tk

class ProgramIsEnding(Exception):
    pass


class UnrecognizedButtonException(Exception):
    pass


class MainWindow(tk.Tk):
    def __init__(self):
        super().__init__()
        self.title('Root Window')
        button = tk.Button(text='Start The Child Window Dialog')
        button.configure(command=self.run_dialog)
        button.grid()
        self.protocol('WM_DELETE_WINDOW', self.delete_window_callback)

    def delete_window_callback(self):
        self.destroy()
        print('Root has been destroyed')
        raise ProgramIsEnding

    def run_dialog(self):
        try:
            button = YesNoDialog(self)()
        except ProgramIsEnding:
            print('Doing end of program stuff.')
            return

        print(f"Button returned is '{button}'")
        if button == 'yes':
            print("'Yes' button clicked")
        elif button == 'no':
            print("'No' button clicked")
        else:
            msg = f"button '{button}'"
            raise UnrecognizedButtonException(msg)


class YesNoDialog:
    window: tk.Toplevel = None
    button_clicked = None

    def __init__(self, parent):
        self.parent = parent

    def __call__(self):
        self.create_window()
        return self.button_clicked

    def create_window(self):
        self.window = tk.Toplevel(self.parent)
        yes = tk.Button(self.window, text='Yes', command=self.yes_command)
        yes.pack(side='left')
        no = tk.Button(self.window, text='No', command=self.no_command)
        no.pack(side='left')
        self.window.wait_window()

    def yes_command(self):
        self.button_clicked = 'yes'
        self.window.destroy()

    def no_command(self):
        self.button_clicked = 'no'
        self.window.destroy()

def main():
    tkroot = MainWindow()
    tkroot.mainloop()


if __name__ == '__main__':
    sys.exit(main())

If the code was working as intended it would terminate correctly having caught the exception, “ProgramIsEnding”. Instead the program terminates with an unhandled “UnrecognizedButtonException”. The full error message follows. Note that the “ProgramIsEnding” exception has been reported via stdout even though it was not available to the try/except handler after control has been handed back to python from Tk.

Root has been destroyed
Exception in Tkinter callback
Traceback (most recent call last):
  File "[…]/python3.7/tkinter/__init__.py", line 1702, in __call__
    return self.func(*args)
  File "[…]/wmdeletedemo.py", line 25, in delete_window_callback
     raise ProgramIsEnding
ProgramIsEnding
Exception in Tkinter callback
Traceback (most recent call last):
  File "[…]/python3.7/tkinter/__init__.py", line 1702, in __call__
    return self.func(*args)
  File "[…]/wmdeletedemo.py", line 41, in run_dialog
    raise UnrecognizedButtonException(msg)
UnrecognizedButtonException: button 'None'
Button returned is 'None'

An obvious workaround is to check if the button value is None and, if so, return. For me, though, good practice suggests that I should check the primary event and neither rely on its secondary effects nor on setting flags.

So, is there some other attribute of Tk or tkinter which records the ending of mainloop or if the root window has been destroyed?

Answer 1

You can set any variable to true inside your delete callback method. Then you can simply check for that variable at the right places. Something like this.

def _delete_window_callback(self):
    """Carry out actions needed when main window is closed."""

    # Save the current geometry
    geometry = self.winfo_geometry()
    config.config_dynamic.set(self.config_section, self.config_geometry, geometry)

    destroyed = true

    # Destroy all widgets and end mainloop.
    self.destroy()

destroyed = false
if destroyed:
    break

Answer 2

The short answer is that Tk does not set an attribute. Tkinter handles this by offering a <Destroy> event for every widget.

The corrected code below has these three features:

• Binding tkinter’s <Destroy> event to the child window’s handler (destroy_callback) insulates the caller and the programmer from any need to understand the various things that might lead to Tk closing the child window. The programmer is thus freed to concentrate on dealing with the effects of the closure.

• A TkClosedWindow exception is raised when YesNoDialog.__call__ exits. This can be handled at any appropriate place in the call chain for MainWindow.run_dialog. If any code which is written later fails to handle the exception the program will fail-fast with an explicit error message.

• The event handler sets a flag to indicate that YesNoDialog.window has been closed. A python exception cannot be raised inside this handler because Tk/Tcl does not propagate python exceptions.

.

import sys
import tkinter as tk


class TkClosedWindow(Exception):
    pass


class UnrecognizedButtonException(Exception):
    pass


class MainWindow(tk.Tk):
    def __init__(self):
        super().__init__()
        self.title('Root Window')
        button = tk.Button(text='Start The Child Window Dialog')
        button.configure(command=self.run_dialog)
        button.grid()

    def run_dialog(self):
        try:
            button = YesNoDialog(self)()
        except TkClosedWindow:
            print('Doing end of program stuff.')
            return

        print(f"Button returned is '{button}'")
        if button == 'yes':
            print("Yes button clicked")
        elif button == 'no':
            print("No button clicked")
        else:
            msg = f"button '{button}'"
            raise UnrecognizedButtonException(msg)


class YesNoDialog:
    window: tk.Toplevel = None
    button_clicked = None
    closed_by_tk = False

    def __init__(self, parent):
        self.parent = parent

    def __call__(self):
        self.create_window()
        if self.closed_by_tk:
            raise TkClosedWindow
        else:
            return self.button_clicked

    def create_window(self):
        self.window = tk.Toplevel(self.parent)
        self.window.bind('<Destroy>', self.destroy_callback)
        yes = tk.Button(self.window, text='Yes', command=self.yes_command)
        yes.pack(side='left')
        no = tk.Button(self.window, text='No', command=self.no_command)
        no.pack(side='left')
        self.window.wait_window()

    def yes_command(self):
        self.button_clicked = 'yes'
        self.window.destroy()

    def no_command(self):
        self.button_clicked = 'no'
        self.window.destroy()

    def destroy_callback(self, *args):
        self.closed_by_tk = True

def main():
    tkroot = MainWindow()
    tkroot.mainloop()


if __name__ == '__main__':
    sys.exit(main())