title: Python nested list (2D array) referencing
body: <p>Looking at my code... why isn't my first nested list (grid1) the same as the 2nd nested list (grid2)? Why is the intilisation different?
  For a game I want a 4 by 6 grid with each element having the value 'x' except grid[0][0] to be assigned to 'O'. This works in <em>grid2</em> but not in <em>grid1</em>. Why is the referencing &amp; list different as it looks the same?</p>
  
  <pre><code>#setup grid1 (4x6 2D nested list) populating x's in all positions
  grid1 =[["x"]*4]*6]
  
  grid1[0][0]="O"      #assign row 1, column 1 'O'
  print(grid1)         
  </code></pre>
  
  <p>outputs</p>
  
  <pre><code>[['O', 'x', 'x', 'x'], ['O', 'x', 'x', 'x'], ['O', 'x', 'x', 'x'], ['O', 'x', 'x', 'x'], ['O', 'x', 'x', 'x'], ['O', 'x', 'x', 'x']]
  </code></pre>
  
  <p>yet this implementation...</p>
  
  <pre><code>grid2 = []  
  for row in range(0,6):
      grid2.append([])
      for column in range(0,4):
          grid2[row].append("x")
  
  grid2[0][0]="O"
  
  print(grid2)
  </code></pre>
  
  <p>outputs correctly</p>
  
  <pre><code>[['O', 'x', 'x', 'x'], ['x', 'x', 'x', 'x'], ['x', 'x', 'x', 'x'], ['x', 'x', 'x', 'x'], ['x', 'x', 'x', 'x'], ['x', 'x', 'x', 'x']]
  </code></pre>
  
  <p>What is the difference &amp; why are they different?</p>