title: Creating a dict from list of key, value tuples while maintaining duplicate keys
body: <p>So I've got a comprehension to the effect of:</p>
  
  <pre><code>dict((x.key, x.value) for x in y)
  </code></pre>
  
  <p>The problem, of course, is that if there's multiple <code>x.key</code>s with the same value, they get collapsed with the last <code>x.value</code> with that particular <code>x.key</code> as the only surviving member. I want to actually make the values of the resulting dict a list:</p>
  
  <pre><code>{
      'key1': ['value1'],
      'key2': ['value2', 'value3', 'value4'],
      'key3': ['value5'],
      # etc.
  }
  </code></pre>
  
  <p>Is this logic possible with a comprehension?</p>