title: len() function does not work for me in iPython Notebook
body: <p>Here is a short example.</p>
  
  <pre><code>    lister = [1,3,5]
      # why doesn't next line work?
      lenl = len(lister)
      # the following line works 
      ll = lister.__len__() 
      print ll
  </code></pre>
  
  <p>Here is the output when I run it:</p>
  
  <p>TypeError                                 Traceback (most recent call last)
   in ()
          1 lister = [1,3,5]
          2 # why doesn't next line work?
    ----> 3 lenl = len(lister)
          4 # the following line works
          5 ll = lister.<strong>len</strong>()</p>
  
  <p>TypeError: 'int' object is not callable</p>