title: How to update a defaultdict with a value of another defaultdict?
body: <pre><code>from collections import defaultdict
  d1 = defaultdict(set)
  d2 = defaultdict(set)
  d1[1].add('abc')
  d1 = {1: {'abc'}}
  
  d2['x'].update(d1) 
  d2 = {'x': {1}}`
  </code></pre>
  <p>but i suppose it to be:</p>
  <pre><code> d2 = {'x': {1: {'abc'}}}
  </code></pre>
  <p>The update function should add a pair, but why it only adds the key. How to achieve that effect I want as above? If I use add function, it gets an ' unhashable type' error.</p>