Kivy Window.on_rotate issue

gistfile1.py

# -*- coding: utf-8 -*-
import kivy
kivy.require('1.8.0')

from kivy.app import App

from kivy.core.window import Window
from kivy.uix.textinput import TextInput
from kivy.uix.boxlayout import BoxLayout

from kivy.properties import ListProperty, ObjectProperty


class MainWindow(BoxLayout):
    "Main screen of the app"

    def do_action(self, action):
        "Called for good or bad button presses"
        self.set_status(action)
        print(Window.size)
        self.orientation = 'vertical'

    def set_status(self, msg):
        self.lbl_status.text = msg


class GoodOrBadApp(App):
    "The main app"

    main_window = None

    def rotate_screen(self, rotation):
        print(rotation)
        self.main_window.set_status(rotation)

    def build(self):
        print(Window.size)
        Window.on_rotate(self.rotate_screen)
        self.main_window = MainWindow()
        return self.main_window


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