title: python list comprehension double for
body: <pre><code>vec = [[1,2,3], [4,5,6], [7,8,9]]
  print [num for elem in vec for num in elem]      &lt;----- this
  
  &gt;&gt;&gt; [1, 2, 3, 4, 5, 6, 7, 8, 9]
  </code></pre>
  
  <p>This is tricking me out.<br>
  I understand elem is the lists inside of the list from <code>for elem in vic</code><br>
  I don't quite understand the usage of <code>num</code> and <code>for num in elem</code> in the beginning and the end.</p>
  
  <p>How does python interpret this?<br>
  What's the order it looks at?</p>