title: weird python dict augement assignment
body: <p>I came across this weird issue this morning:</p>
  
  <pre><code>&gt;&gt;&gt; l = ['a', 'b', 'c']
  &gt;&gt;&gt; dict = dict.fromkeys(l, [])
  &gt;&gt;&gt;　dict['a'] += [1]
  &gt;&gt;&gt; dict
  {'a': [1], 'c': [1], 'b': [1]}
  </code></pre>
  
  <p>I can not explain why this happens?</p>