Q:

How to run a Method on the exit of a kivy app

I would like to run a Method when the user tries to exit the app , kind of like a "are you sure you want to exit" or "Do you want to save the file" type of message whenever the user tries to exit by clicking the Exit button on top of the window
Some thing like 
on_quit: app.root.saveSession()

A:

If you want your application to simply run things after the GUI has closed, the easiest and smallest approach would be to place any exit code after TestApp().run().  run() creates a endless loop which also clears any event-data from within kivy so it doesn't hang.  That endless loop breaks as soon as the window/gui instance dies.  So there for, any code after will execute only after the GUI dies too. 
If you want to create a graceful shutdown of the GUI with for instance socket-closing events or a popup asking the user if that's what they really want to do, then creating a hook for the on_request_close event is the way to go:
from kivy.config import Config
Config.set('kivy', 'exit_on_escape', '0')

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.popup import Popup
from kivy.core.window import Window

class ChildApp(App):

    def build(self):
        Window.bind(on_request_close=self.on_request_close)
        return Label(text='Child')

    def on_request_close(self, *args):
        self.textpopup(title='Exit', text='Are you sure?') 
        return True

    def textpopup(self, title='', text=''):
        """Open the pop-up with the name. 

        :param title: title of the pop-up to open
        :type title: str
        :param text: main text of the pop-up to open
        :type text: str
        :rtype: None
        """
        box = BoxLayout(orientation='vertical')
        box.add_widget(Label(text=text))
        mybutton = Button(text='OK', size_hint=(1, 0.25))
        box.add_widget(mybutton)
        popup = Popup(title=title, content=box, size_hint=(None, None), size=(600, 300))
        mybutton.bind(on_release=self.stop)
        popup.open()

if __name__ == '__main__':
    ChildApp().run()

Courtesy of pythonic64 who created a gist on the topic in a issue way back when.