title: python the list in dict extend value
body: <p>How do I create a dictionary of lists? Here is what I tried:</p>
  <pre class="lang-py prettyprint-override"><code>zigzag = dict.fromkeys(range(1, 4),[])
  for i in range(1, 4):
      zigzag[i].extend(range(i, 13 , 4))
  </code></pre>
  <p>What I expected to get was:</p>
  <pre class="lang-py prettyprint-override"><code>{
    1: [1,5,9,13],
    2: [2,6,10],
    3: [3,7,11]
  }
  </code></pre>
  <p>But the actual result had the same list in each value:</p>
  <pre class="lang-py prettyprint-override"><code>{
    1: [1, 5, 9, 2, 6, 10, 3, 7, 11],
    2: [1, 5, 9, 2, 6, 10, 3, 7, 11],
    3: [1, 5, 9, 2, 6, 10, 3, 7, 11]
  }
  </code></pre>
  <p>I don't know how that result happened. How can I get my expected answer?</p>