如何从.py设置.kv中可选标签的文本

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

class MyLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyLayout, self).__init__(**kwargs)
        
        # 创建标签
        self.label = Label(text="默认文本")
        self.add_widget(self.label)
        
        # 创建按钮
        self.button = Button(text="更改文本", on_press=self.change_text)
        self.add_widget(self.button)
        
    def change_text(self, instance):
        # 当按钮被点击时，更改标签的文本
        self.label.text = "新的文本"

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

<MyLayout>:
    orientation: "vertical"
    padding: 10
    
    Label:
        text: root.label.text
        font_size: 20
        
    Button:
        text: "更改文本"
        on_press: root.change_text(self)

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