title: python dict update diff
body: <p>Does python have any sort of built in functionality of notifying what dictionary elements changed upon dict update?  For example I am looking for some functionality like this:</p>
  
  <pre><code>&gt;&gt;&gt; a = {'a':'hamburger', 'b':'fries', 'c':'coke'}
  &gt;&gt;&gt; b = {'b':'fries', 'c':'pepsi', 'd':'ice cream'}
  &gt;&gt;&gt; a.diff(b)
  {'c':'pepsi', 'd':'ice cream'}
  &gt;&gt;&gt; a.update(b)
  &gt;&gt;&gt; a
  {'a':'hamburger', 'b':'fries', 'c':'pepsi', 'd':'ice cream'}
  </code></pre>
  
  <p>I am looking to get a dictionary of the changed values as shown in the result of a.diff(b)  </p>