title: Python dictionary keys besides strings and integers?
body: <p>Anyone have some neat examples of dictionaries with some interesting keys (besides the canonical string or integer), and how you used these in your program?</p>
  
  <p>I understand all we need for a key is something <code>hashable</code>, meaning it must be immutable and comparable (has an <code>__eq__()</code> or <code>__cmp__()</code> method). </p>
  
  <p>A related question is: how can I quickly and slickly define a new <code>hashable</code>?</p>