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I'm still uncertain about what to do on 301 responses.  As
you say, there are two issues to sort out: 1) what method
should a POST be redicted to and 2) whether the user should
be asked to confirm.

There's discussion of this at:
http://ppewww.ph.gla.ac.uk/~flavell/www/post-redirect.html
The page is a bit old, but I don't know if it is out of date.
It says that IE5, Netscape 4, and Mozilla 1.0 all redirect a
301 POST to a GET without user confirmation.  That seems
like a widespread disregard for the spec that we ought to
emulate.

I agree with your interpretation that urllib2 raise an
HTTPError to signal "request confirm" because an HTTPError
is also a valid response that the user could interpret.  But
of urllib, an HTTP error doesn't contain a valid response. 
The change would make it impossible for the client to do
anything if a 301 response is returned from a POST.  That
seems worse than doing the wrong.