title: what is the 'self.__class__.__missing__' mean
body: <p>in pinax Userdict.py:</p>
  
  <pre><code>def __getitem__(self, key):
          if key in self.data:
              return self.data[key]
          if hasattr(self.__class__, "__missing__"):
              return self.__class__.__missing__(self, key)
  </code></pre>
  
  <p>why does it do this on <code>self.__class__.__missing__</code>.</p>
  
  <p>thanks</p>