How can one mock/stub python module like urllib

I need to test a function that needs to query a page on an external server using urllib.urlopen (it also uses urllib.urlencode). The server could be down, the page could change; I can't rely on it for a test. What is the best way to control what urllib.urlopen returns?

[aroma_black]

urllib2のテストの仕方。動的言語の特性をうまく使ってる。