title: Kivy official Pong tutorial - usage of Vector (kivy.vector)
body: <p>I've been following official Kivy PongApp tutorial (<a href="http://kivy.org/docs/tutorials/pong.html" rel="noreferrer">link</a> - whole program code at the bottom of site) and I've faced a problem I can't really understand.</p>
  
  <p>I've defined the move function to change the position of the ball by a velocity vector on each frame. The code:</p>
  
  <pre><code>def move(self):
      self.pos = Vector(*self.velocity) + self.pos
  </code></pre>
  
  <p>However, when I've written the code like this:</p>
  
  <pre><code>def move(self):
      self.pos = self.pos + Vector(*self.velocity)
  </code></pre>
  
  <p>It results in an error:
  ValueError: PongBall.pos value length is immutable</p>
  
  <p>Why, shouldn't it be the same?</p>