The version I used is :

>>> sys.version
'2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 
bit (Intel)]'

On Windows XP SP2.

While I think every python 2.5 releases are concerned.

On line 357 of urllib.py from 2.5 release, there is a 
raise of an IOError with 4 arguments. It look like it 
was fine with python 2.4 but it hang up with a 
"TypeError: EnvironmentError expected at most 3 
arguments, got 4"

Concretly, when you hit a page with a "redirect" using 
error 302 for exemple, instead of raising an IOError, 
it raise a TypeError, so it break code which expect an 
IOError here (as a "normal" behavior for 302 codes)

It look like IOError is totally different between 
Python 2.4 and Python 2.5 (it was a class, it's now a 
type)