您需要返回一个根部件。

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

kv = '''
<MyKivyRoot>:
    Label:
        text: root.show_text()
    NewClass

<NewClass>:
    Label:
        text: app.root.show_text() if app.root else ''

# The first two just define classes, they don't create instances:
MyKivyRoot:
# This does
'''

class MyKivyRoot(BoxLayout):
    def show_text(self):
        return "Here is your text"

class NewClass(BoxLayout):
    pass

class MyKivyApp(App):
    def build(self):
        return Builder.load_string(kv)

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

您需要在根小部件声明中声明子小部件。尖括号表示法用于定义小部件，但它们也必须声明。用户定义的变量必须在小部件定义中声明，以便它们可以作为小部件声明的参数传递。

#untitled.kv#

Screen:
    FooWidget:
        name: 'World'
        size_hint: 1, .5
        pos_hint: {'top': 1}
    BarWidget:
        color: 1, .5, 0
        size_hint: 1, .5
        pos_hint: {'y': 0}

<FooWidget@Label>:
    name: ''
    text: f'Hello, {self.name}'

<BarWidget@Widget>:
    color: 0, 0, 0
    canvas.before:
        Color: 
            rgb: root.color
        Rectangle:
            pos: root.pos
            size: root.size

-------------------------------------
#untitled.py#

from kivy.app import App

class UntitledApp(App):
    pass

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