title: Beginner python set intersection error
body: <pre><code>rare = (["word1","word4","word5"])
  freq = (["word1","word2","word3"])
  unique = rare.intersection(freq)
  print unique
  </code></pre>
  
  <p>error: AttributeError: 'list' object has no attribute 'intersection'</p>
  
  <p>Am I not creating the sets correctly?  They look like the examples in documentation -- but I can't seem to use normal set methods on them.</p>
  
  <p>What is the proper syntax for creating sets if these are lists?</p>