title: defaultdict appending list of list rather than list?
body: <p>I am trying to use collections.defaultdict in python 3.
  I tried following steps in console:</p>
  
  <pre><code>&gt;&gt;&gt; from collections import defaultdict
  &gt;&gt;&gt; l = [1,2,3,4,5]
  &gt;&gt;&gt; dd = defaultdict(list)
  &gt;&gt;&gt; dd["one"].append(l)
  &gt;&gt;&gt; print(dd)
  defaultdict(&lt;class 'list'&gt;, {'one': [[1, 2, 3, 4, 5]]})
  </code></pre>
  
  <p>As, you can see its adding <code>[[1, 2, 3, 4, 5]]</code> i.e. list of list so I need two for loops to read its variables.</p>
  
  <p>Why it is not appending something like <code>[1, 2, 3, 4, 5]</code>??</p>
  
  <p>Is there something wrong with my implementation or understanding how defaultdict works?
  Thank you in advance</p>