如何更新TextInput小部件中显示的内容？python kivy

from kivy.app import App
from kivy.uix.textinput import TextInput
from kivy.uix.boxlayout import BoxLayout

class MyBoxLayout(BoxLayout):
    def update_text(self):
        # 获取TextInput小部件
        text_input = self.ids.my_text_input
        # 修改text属性
        text_input.text = "新的内容"

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

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