title: difference in methods of declaring two dimensional array
body: <p>There are two methods exists to declare a 2D array in python, 
  first one is </p>
  
  <pre><code>matrix = [[0.0 for x in range(20)] for x in range(41)] 
  </code></pre>
  
  <p>second is,</p>
  
  <pre><code>matrix = [[0.0] * 20] * 41
  </code></pre>
  
  <p>The first declaration is working fine, but the second declaration is generating unnecessary references. How is this possible?</p>