title: "Map" a nested list in Python
body: <p>In Python, I am trying to apply an operator to a two layer nested array. For example, </p>
  
  <pre><code>a = [['2.3','.2'],['-6.3','0.9']]
  for j in range(2)
      for i in range(2)
          a[i][j] = float(a[i][j])
  </code></pre>
  
  <p>How can I do this without the loops? I am hoping for something akin to a= map(float,a). Of course the last script does not work for a nested list. A one line list comprehension may be acceptable too.</p>