如何与kivy切换屏幕？

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.button import Button
from kivy.uix.label import Label


class MainScreen(Screen):
    def __init__(self, **kwargs):
        super(MainScreen, self).__init__(**kwargs)
        self.layout = BoxLayout(orientation='vertical')
        self.label = Label(text='Main Screen')
        self.button = Button(text='Go to Second Screen', on_press=self.switch_screen)
        self.layout.add_widget(self.label)
        self.layout.add_widget(self.button)
        self.add_widget(self.layout)

    def switch_screen(self, instance):
        self.manager.current = 'second'


class SecondScreen(Screen):
    def __init__(self, **kwargs):
        super(SecondScreen, self).__init__(**kwargs)
        self.layout = BoxLayout(orientation='vertical')
        self.label = Label(text='Second Screen')
        self.button = Button(text='Go to Main Screen', on_press=self.switch_screen)
        self.layout.add_widget(self.label)
        self.layout.add_widget(self.button)
        self.add_widget(self.layout)

    def switch_screen(self, instance):
        self.manager.current = 'main'


class MyScreenManager(ScreenManager):
    def __init__(self, **kwargs):
        super(MyScreenManager, self).__init__(**kwargs)
        self.add_widget(MainScreen(name='main'))
        self.add_widget(SecondScreen(name='second'))


class MyApp(App):
    def build(self):
        return MyScreenManager()


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