如何更新kivy的对话文本

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label

class MyApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        self.label = Label(text="初始文本")
        layout.add_widget(self.label)
        return layout

from kivy.uix.button import Button

class MyApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        self.label = Label(text="初始文本")
        layout.add_widget(self.label)

        self.button = Button(text="点击更改文本")
        self.button.bind(on_press=self.update_label_text)
        layout.add_widget(self.button)

    def update_label_text(self, instance):
        self.label.text = "新文本"