title: Python collections.MappingView
body: <p>I was checking out the very nice collections library and more specific the Abstract Base Classes (ABC). One I could not get my head around: the MappingView.</p>
  
  <ul>
  <li>What is its use? What is its advantage over Sized? An example perhaps?</li>
  <li>Documentation says its base class is Sized, but on the other hand there is a <strong>len</strong> mixin... So do we have to implement <strong>len</strong>, or not?</li>
  </ul>
  
  <p>For the documentation, see <a href="http://docs.python.org/2/library/collections.html#collections-abstract-base-classes" rel="noreferrer">collections</a></p>