title: Python doctests: test for None
body: <p>Using Python 2.7 I'm trying to test that the result of a particular function call is None</p>
  
  <p>I would expect these tests to pass (excuse the rather silly example)</p>
  
  <pre><code>def six_or_none(val):
      """
      &gt;&gt;&gt; six_or_none(6)
      6
      &gt;&gt;&gt; six_or_none(4)
      None
      """
      if val == 6:
          return 6
      return None
  </code></pre>
  
  <p>However they yield the following result</p>
  
  <pre><code>Failed example:
      six_or_none(4)
  Expected:
      None
  Got nothing
  </code></pre>
  
  <p>What's the correct way to test for None in doctests?</p>