title: Unexpected Behavior of Extend with a list in Python
body: <p>I am trying to understand how extend works in Python and it is not quite doing what I would expect.  For instance:</p>
  
  <pre><code>&gt;&gt;&gt; a = [1, 2, 3]
  &gt;&gt;&gt; b = [4, 5, 6].extend(a)
  &gt;&gt;&gt; b
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>But I would have expected:</p>
  
  <pre><code>[4, 5, 6, 1, 2, 3]
  </code></pre>
  
  <p>Why is that returning a None instead of extending the list?</p>