How to dismiss the Kivy pop-up via a Button?

I have a pop-up created with Kivy, which contains 2 buttons. User can dismiss the pop-up by pressing outside of the pop-up area (auto_dismiss = True), or by clicking the "No" button. Selecting the "Yes" button, will exit the whole application.

Please see relevant code:

class ExitApp(App):

def exit_confirmation(self):

    # popup can only have one Widget.  This can be fixed by adding a BoxLayout

    self.box_popup = BoxLayout(orientation = 'horizontal')

    self.box_popup.add_widget(Label(text = "Really exit?"))

    self.box_popup.add_widget(Button(
        text = "Yes",
        on_press = ExitApp.exit,
        size_hint = (0.215, 0.075)))

    self.box_popup.add_widget(Button(
        text = "No",
        on_press = self.popup_exit.dismiss,
        size_hint=(0.215, 0.075)))

    self.popup_exit = Popup(title = "Exit",
        content = self.box_popup,
        size_hint = (0.4, 0.4),
        auto_dismiss = True)

    self.popup_exit.open()

def exit(self):

    App.get_running_app().stop()

The problem now lays with pressing the "No" button. When that is pressed, the code exits with this error:

 on_press = self.popup_exit.dismiss,

AttributeError: 'Button' object has no attribute 'popup_exit'

Any idea how I can fix this as easily as possible?

You can solve this issue by a lazy function

on_press = lambda *args: self.popup_exit.dismiss()

This way, the lookup will occur only when the button is pressed and popup_exit is already in place...