first_screen.kv

<FirstScreen>:
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'This is the first screen.'
        Button:
            text: 'This button prints a number.'
            on_press: root.print_number()  # In a custom widget's Kv, `root` refers to the custom widget.
        Button:
            text: 'Change…'
            on_press: app.open_second_screen()  # But `app` still refers to the application object.