title: average of attribute in a list of objects
body: <p>I have a list of user's. Each user will have a <code>waitTime</code> attribute. How do I get the average <code>waitTime</code> for all users in the list? The code below will probably work, but I'm guessing that there is a better way to do it.</p>
  
  <pre><code>sum = 0
  for user in self.done:
      sum += user.waitTime 
  sum/len(self.done)
  </code></pre>