title: Understanding python list comprehension
body: <p>I was seeking a solution for <a href="https://stackoverflow.com/questions/3678869/pythonic-way-to-combine-two-lists-in-an-alternating-fashion">Pythonic way to combine two lists in an alternating fashion?</a> And I came across this elegant solution by @Someone.</p>
  <pre><code>[x for y in zip(list1, list2) for x in y]
  </code></pre>
  <p>It works like a charm, but I do not understand how the variables <code>x</code> and <code>y</code> are being assigned here. What is really going on in the list comprehension?</p>