title: Transform a collection of key-value pairs into a dictionary that lists the values?
body: <p>I'm confident there's a succinct pythonic way to convert:</p>
  
  <pre><code>[('a', 1), ('a', 2), ('b', 4)]
  </code></pre>
  
  <p>Into:</p>
  
  <pre><code>{'a':[1, 2], 'b':[4]}
  </code></pre>
  
  <p>I've tried various combinations of <code>zip</code>, <code>dict</code>, and <code>itertools.groupby</code> and I can't get my head round how to do this. </p>