title: Iterating thru list for lxml form submission
body: <p>I'm pretty new to Python and even more new to lxml, but what I'm trying to do seems really simple but I can't figure out what I'm doing wrong.</p>
  
  <p>I have this code with the goal of feeding a list of values (list object ISBN) to lxml to submit to a search field:</p>
  
  <pre><code>for i in ISBN:
  
      page.forms[0].fields['_nkw'] = ISBN[i]
      blah blah blah
  </code></pre>
  
  <p>I get this error after running:  </p>
  
  <pre><code>Traceback (most recent call last):
  
  page.forms[0].fields['_nkw'] = ISBN[i]
  TypeError: list indices must be integers, not str 
  </code></pre>
  
  <p>Obviously there has to be a way to iterate through a list of values to feed to a form, but clearly I don't know it :)</p>
  
  <p>EDIT: FYI the code works fine when replacing ISBN[i] with hard input.
  EDIT 2: contents of ISBN list object as requested:</p>
  
  <pre><code>['9781608319053', '9780321558237', '9781932735413', '9781416059516', '9781437708257', '9780781780582', '9781437701517', '9780323065801', '9780890420256', '9780323079334', '9781599417042', '9780781771535', '9781416031215', '9780312601430', '9780781775250', '9781591263333', '9780071748896', '9780133669510', '9781416045748', '9780781771566', '9781437728019', '9780323065849', '9781416066675', '9780735579965', '9780323078917', '9781437735826', '9781603595681', '9780321696724', '9780321558145', '9781933107981', '9780138024611']
  </code></pre>