title: Python: first element in a nested list
body: <p>I would like a list that contains only the first elements of the nested list.
  The nested list L, it's look like:</p>
  
  <pre><code>L =[ [[0,1,2],[3,4,5],[6,7,8]], [[0,1,2],[3,4,5],[6,7,8]], [[0,1,2],[3,4,5],[6,7,8]] ]
  
  for l in L:
   for t in l:
    R.append(t[0])
  print 'R=', R
  </code></pre>
  
  <p>The Output is <code>R= [0, 3, 6, 0, 3, 6, 0, 3, 6]</code> but I want to get a separated result like:</p>
  
  <pre><code>R= [[0, 3, 6], [0, 3, 6], [0, 3, 6]]
  </code></pre>
  
  <p>I've also tried through a list comprehension like <code>[[R.append(t[0]) for t in l] for l in L]</code> but this gives <code>[[None, None, None], [None, None, None], [None, None, None]]</code></p>
  
  <p>What is wrong?</p>