title: Count number of elements in a iterable
body: <p>What's the best way of counting the number of elements in a iterable (through iteration), without storing the elements in memory?</p>
  
  <p>Right now, I'm just doing <code>sum(1 for x in iterable)</code>. I was hoping to find something in <code>itertools</code>, but there doesn't seem to be anything.</p>