title: Grouping items in Python Dictionary by common value
body: <p>I have a nested dictonary that I'm iterating over, I'd like to make a new dictonary derived from the old dictonry that groups certain values together based on a value present in the old dictonary. To illustrate:</p>
  
  <pre><code>{'name': Fido, 'breed': Dalmatian, 'age': 3}
  {'name': Rex, 'breed': Dalmatian, 'age': 2}
  {'name': Max, 'breed': Dalmatian, 'age': 0}
  {'name': Rocky, 'breed': Pitbull, 'age': 6}
  {'name': Buster, 'breed': Pitbull, 'age': 7}
  </code></pre>
  
  <p>Would give me:</p>
  
  <pre><code>Dalmation: {'name': [Fido, Rex, Max], 'age': [3, 2, 0]}
  Pitbull : {'name': [Rocky, Buster], 'age': [6, 7]} 
  </code></pre>
  
  <p>I've tried to find an elegant and pythonic solution to this to no avail.</p>