kivy/examples/tutorials/pong/steps/step3/main.py

import kivy
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import NumericProperty, ReferenceListProperty
from kivy.vector import Vector
from kivy.factory import Factory

class PongBall(Widget):
    velocity_x = NumericProperty(0)
    velocity_y = NumericProperty(0)
    velocity = ReferenceListProperty(velocity_x, velocity_y)
    
    def move(self):
        self.pos = Vector(*self.velocity) + self.pos


class PongGame(Widget):
    pass


class PongApp(App):
    def build(self):
        return PongGame()


Factory.register("PongBall", PongBall)
if __name__ == '__main__':
    PongApp().run()