如何在Kivy中只做一次add_widget

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

class MyBoxLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyBoxLayout, self).__init__(**kwargs)
        self.orientation = 'vertical'

def add_single_widget(self, widget):
    self.clear_widgets()  # 清空布局中的widget
    self.add_widget(widget)  # 添加指定widget到布局

class MyApp(App):
    def build(self):
        layout = MyBoxLayout()
        button = Button(text='Hello World')
        layout.add_single_widget(button)  # 调用自定义布局方法添加widget
        return layout

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