如何从screen实例外部切换Kivy Python文件中的屏幕

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

class HomeScreen(Screen):
    pass

class SettingsScreen(Screen):
    pass

sm = ScreenManager()
sm.add_widget(HomeScreen(name='home'))
sm.add_widget(SettingsScreen(name='settings'))

class HomeScreen(Screen):
    def __init__(self, **kwargs):
        super(HomeScreen, self).__init__(**kwargs)
        self.add_widget(Label(text='Welcome to Home Screen'))

class SettingsScreen(Screen):
    def __init__(self, **kwargs):
        super(SettingsScreen, self).__init__(**kwargs)
        self.add_widget(Label(text='Welcome to Settings Screen'))

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

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

class HomeScreen(Screen):
    def __init__(self, **kwargs):
        super(HomeScreen, self).__init__(**kwargs)
        self.add_widget(Label(text='Welcome to Home Screen'))
        self.add_widget(Button(text='Go to Settings', on_release=self.go_to_settings))

    def go_to_settings(self, instance):
        sm.current = 'settings'