kivy中的屏幕管理器问题

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


class LoginScreen(Screen):
    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.add_widget(Button(text='Go to Home', on_release=self.go_to_home))

    def go_to_home(self, *args):
        self.manager.current = 'home'


class HomeScreen(Screen):
    def __init__(self, **kwargs):
        super(HomeScreen, self).__init__(**kwargs)
        self.add_widget(Button(text='Go to Login', on_release=self.go_to_login))

    def go_to_login(self, *args):
        self.manager.current = 'login'


class MyApp(App):
    def build(self):
        screen_manager = ScreenManager()
        screen_manager.add_widget(LoginScreen(name='login'))
        screen_manager.add_widget(HomeScreen(name='home'))
        return screen_manager


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