from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import (
    NumericProperty, ReferenceListProperty, ObjectProperty
)
from kivy.vector import Vector
from kivy.clock import Clock
from random import randint
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):
    ball = ObjectProperty(None)
    def serve_ball(self):
        self.ball.center = self.center
        self.ball.velocity = Vector(4, 0).rotate(randint(0, 360))
    def update(self, dt):
        self.ball.move()
        if(self.ball.y < 0) or (self.ball.top > self.height):
            self.ball.velocity_y *= -1
        if(self.ball.x < 0) or (self.ball.right > self.width):
            self.ball.velocity_x *= -1        
class PongApp(App):
    def build(self):
        game = PongGame()
        game.serve_ball()
        Clock.schedule_interval(game.update, 1.0/60)
        return game
if __name__ == '__main__':
    PongApp().run()

 class Foo(object):
    @property
    def foo(self):
        return 0
f = Foo()
print(f.__dict__)