title: Extend a list with another one. Why this behaviour?
body: <p>I have a 2 elements list which I want to extend with the elements of another one.
  playing around in Jupyter I found this strange behaviour that I can not understand.</p>
  
  <pre><code># this is my first list
  columnslist=['app','SUM']
  ['app','SUM']
  
  # it is going to be extended with the values A and B
  
  columnslist.extend(['A','B'])
  ['app','SUM','A','B']
  
  # It is possible to make a list out of a string in this way
  print(list('AB'))
  ['A','B']
  
  # I realise that list('AB')= ['A','B']
  This works:
  columnslist.extend(list('AB'))
  
  # but the following does not work:
  mytext='this is a text to be split'
  columnslist.extend(mytext.split())
  </code></pre>
  
  <p>why is that so? 
  thanks</p>