title: Adding a list to an empty python list issue
body: <p>I am Python beginner with no prior programming experience.
  I apologize in advance if this question sounds too lame to answer to, but there is something I am confused with. It's related to adding a particular list (l2) to an empty list (l1).
  What I am trying to do is to increase the value of each of the "l2" elements in each iteration, and then add it to the l2 list.Here is the code:</p>
  
  <pre><code>l1 = []
  l2 = [0,1,2,3]
  
  for i in range(3):
      l2[0] = l2[0] + 1
      l2[1] = l2[1] + 1
      l2[2] = l2[2] + 1
      l2[3] = l2[3] + 1
      print l2
      l1.append(l2)
  
  print l1
  </code></pre>
  
  <p>This is the print result that I get:</p>
  
  <pre><code>[1, 2, 3, 4]
  [2, 3, 4, 5]
  [3, 4, 5, 6]
  [[3, 4, 5, 6], [3, 4, 5, 6], [3, 4, 5, 6]]
  </code></pre>
  
  <p>Why doesn't l2 list look like this:</p>
  
  <pre><code>[[1, 2, 3, 4], [2, 3, 4, 5], [3, 4, 5, 6]]
  </code></pre>
  
  <p>?
  Thank you for the reply.</p>