title: How to convert defaultdict to dict?
body: <p>How can I convert a <code>defaultdict</code></p>
  <pre><code>number_to_letter
  defaultdict(&lt;class 'list'&gt;, {'2': ['a'], '3': ['b'], '1': ['b', 'a']})
  </code></pre>
  <p>to be a common <code>dict</code>?</p>
  <pre><code>{'2': ['a'], '3': ['b'], '1': ['b', 'a']}
  </code></pre>