Kivy: separation of UI and App

main.py

from kivy.app import App
from kivy.properties import StringProperty

class TestApp(App):
    appabc = StringProperty('')

    def build(self):
        self.bind(appabc=self.root.setter('abc'))
        self.appabc = 'hello'

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

test.kv

BoxLayout:
    abc: ''
    Label:
        text: root.abc