如何在kv文件中使用相同的小部件

<BaseWidget>:
    Label:
        text: "This is a base widget"

<CustomWidget>:
    BaseWidget:
    Label:
        text: "This is a custom widget"

from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.app import App

Builder.load_file('base.kv')
Builder.load_file('custom.kv')

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

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