title: Python defaultdict reference
body: <p>Actually, I don't know how to explain this question in a proper title. Any edition is welcome.</p>
  
  <p>Let's just see the example.</p>
  
  <pre><code># python 2.7.x
  import collections
  d = collections.defaultdict(int)
  d['a'] = 2
  d['b'] = 1
  res = [d]* 2
  res[0]['a'] -= 1
  print res[1]
  # =&gt; defaultdict(&lt;type 'int'&gt;, {'a': 1, 'b': 1})
  </code></pre>
  
  <p>I was wondering why it affects the <code>res[1]</code>? </p>