title: fors in python list comprehension
body: <p>Consider I have nested for loop in python list comprehension</p>
  
  <pre><code>&gt;&gt;&gt; data = [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
  &gt;&gt;&gt; [y for x in data for y in x]
  [0, 1, 2, 3, 4, 5, 6, 7, 8]
  &gt;&gt;&gt; [y for y in x for x in data]
  [6, 6, 6, 7, 7, 7, 8, 8, 8]
  </code></pre>
  
  <p>I can't explain why is that when i change the order of two <code>fors</code> </p>
  
  <pre><code>for y in x
  </code></pre>
  
  <p>What's <code>x</code> in second list comprehension?</p>