title: how to parse iframe in lxml in python?
body: <p>I found that lxml can't parse heml element of iframe .</p>
  
  <pre><code>import lxml.html
  from urllib.request import urlopen
  import os
  url="http://news.163.com/special/mhmingdan/?bdsj"
  file=urlopen(url).read()
  root=lxml.html.document_fromstring(file)
  tab=root.xpath('//iframe')
  </code></pre>
  
  <p>How can make lxml get the html element of iframe?</p>