title: Please explain what happens here: b=[x[:] for x in a]
body: <p>I don't quite understand what is happening in the below code. Is it creating a copy of a nested list? If so, how is it doing it? What is b equal to?</p>
  
  <pre><code>a = [[1,2,3],[4,5,6]]
  
  b = [x[:] for x in a]
  </code></pre>