title: Usage of ObjectProperty class
body: <p>I Just started to learn kivy and I am very confused on the usage of the ObjectProperty class, and how it takes None as an argument. Could somebody explain it please? I found it in the kivy tutorial:</p>
  
  <pre><code>class PongGame(Widget):
      ball = ObjectProperty(None)
  
      def update(self, dt):
          self.ball.move()
  
          # bounce off top and bottom
          if (self.ball.y &lt; 0) or (self.ball.top &gt; self.height):
              self.ball.velocity_y *= -1
  
          # bounce off left and right
          if (self.ball.x &lt; 0) or (self.ball.right &gt; self.width):
              self.ball.velocity_x *= -1
  </code></pre>