HTTP response capturing issue

[miunika]

I want to catch a specific HTTP error and not any one of the entire family.. what I was trying to do is
import urllib2

try:
   urllib2.urlopen("some url")
except urllib2.HTTPError:
   <whatever>

but what I end up is catching any kind of HTTP error, but I want to catch only if the specified webpage doesn't exist!! probably that's HTTP error 404..but I don't know how to specify that catch only error 404 and let the system run the default handler for other events.