gistfile1.txt

<Player>:
    canvas:
        Rectangle:
            pos: root.center
            size: 40, 40

<Screen>:
    Player

gistfile2.txt

from kivy.app import App
from kivy.core.window import Window
from kivy.uix.widget import Widget
from kivy.properties import ListProperty

class Player(Widget):
    pass

class Screen(Widget):
    pass

class GameApp(App):
    def build(self):
        return Screen()

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