title: List of lists changes reflected across sublists unexpectedly
body: <p>I created a list of lists:</p>
  <pre><code>xs = [[1] * 4] * 3
  
  # xs == [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]  
  </code></pre>
  <p>Then, I changed one of the innermost values:</p>
  <pre><code>xs[0][0] = 5
  
  # xs == [[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]  
  </code></pre>
  <p>Why did every first element of each sublist change to <code>5</code>?</p>