title: "None" string appear after print a line in python
body: <p>I a new in python programming. Today I practice with sorting a list. 
  I've got a strange output with this code:</p>
  
  <pre><code># problem 13
  def lensort(list):
     list.sort(key=lambda x: len(x))
     print list 
  
  print "Sort by length: ", lensort(['python', 'perl', 'java', 'c', 'haskell','ruby'])
  </code></pre>
  
  <p>The output is: </p>
  
  <pre><code>Sort by length:  ['c', 'perl', 'java', 'ruby', 'python', 'haskell']
  None
  </code></pre>
  
  <p>So, I have no idea the reason why "None" is shown in 2nd line.
  Could anyone help to about this problem?</p>
  
  <p>Thank you very much!</p>