title: python conditional constructor for giving object or None
body: <p>I receive UDP packets with GPS data, and construct Position objects with them.
  Sometimes, GPS loss satellite signal, but still send UDP packets with no valuable data. I would like the Position constructor gives me a 'None' object in this failure cases. So, I could check if process the Position or not. Something like that:</p>
  
  <pre><code>frameGPS = giveMeTheFrame()
  position1 = Position(frameGPS)
  if (position1 is None): don't process position1
  else: process position1
  </code></pre>
  
  <p>Is this a right way to do it?
  If so, how could I implement this <strong>init</strong>?</p>