title: Mutable namedtuple?
body: <p>Suppose I have:</p>
  
  <pre><code>from collections import namedtuple
  NT = namedtuple('name', ['x'])
  </code></pre>
  
  <p>Can someone explain the difference between:</p>
  
  <ol>
  <li><code>NT.x = 3</code> </li>
  <li><code>var = NT(x=3)</code>  </li>
  </ol>
  
  <p>I can change <code>NT.x</code> to anything (mutable) but <code>var.x</code> is immutable. Why is that the case?</p>