main.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.lang import Builder

Builder.load_string("""
<RootWidget>:
	Label:
		text: 'Hello World'
		size: root.size
		pos: root.pos
        """)

class RootWidget(Widget):
    pass

class HelloWorldApp(App):

    def build(self):
        return RootWidget()

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