title: Adding extra fields to an existing namedtuple object and unpickling
body: <p>I have some data in the pickled format with instances of below objects:</p>
  
  <pre><code>Point = namedtuple('Point', ['x',])
  </code></pre>
  
  <p>Now I want to extend the Point object to add another variable 'y' but also keep this compatible with the data I already have pickled.</p>
  
  <p>Below is what I tried, but seems to fail. I also tried playing around with the code to make a Point class by setting the y parameter as optional when making the Point object, but that did not work as well. Any ideas on how to proceed?</p>
  
  <pre><code>from collections import namedtuple
  Point = namedtuple('Point', ['x'])
  i = Point(1)
  import cPickle
  pick=cPickle.dumps(i)
  Point = namedtuple('Point', ['x', 'y'])
  cPickle.loads(pick)
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "&lt;string&gt;", line 22, in __repr__
  TypeError: not enough arguments for format string
  </code></pre>