Issues a longer xpath expression

[python]

I am having issues with the urllib and lxml.html modules.

Here is my original code:

import urllib  
import lxml.html  
down='http://v.163.com/special/visualizingdata/'  
file=urllib.urlopen(down).read()  
root=lxml.html.document_fromstring(file)
xpath_str="//div[@class='down s-fc3 f-fl']/a"  
urllist=root.xpath(xpath_str)for url in urllist:print url.get("href")

When run, it returns this output:

http://mov.bn.netease.com/movieMP4/2012/12/A/7/S8H1TH9A7.mp4  
http://mov.bn.netease.com/movieMP4/2012/12/D/9/S8H1ULCD9.mp4  
http://mov.bn.netease.com/movieMP4/2012/12/4/P/S8H1UUH4P.mp4  
http://mov.bn.netease.com/movieMP4/2012/12/B/V/S8H1V8RBV.mp4  
http://mov.bn.netease.com/movieMP4/2012/12/6/E/S8H1VIF6E.mp4  
http://mov.bn.netease.com/movieMP4/2012/12/B/G/S8H1VQ2BG.mp4  

But, when I change the line

xpath_str='//div[@class="down s-fc3 f-fl"]//a'

into

xpath_str='//div[@class="col f-cb"]//div[@class="down s-fc3 f-fl"]//a'

that is to say,

urllist=root.xpath('//div[@class="col f-cb"]//div[@class="down s-fc3 f-fl"]//a')

I do not receive any output. What is the flaw in this code?
it is so strange that the shorter one can work,the longer one can not,they have the same xpath structure!
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