URLLIB2 seems to have issues attempting to digest authenticate against a
Windows-based IIS 6.x server.  The IIS server requests MD5-sess support,
and URLLIB2 throws an exception that ... far from explains this:

Traceback (most recent call last):
  File "/usr/lib64/python2.5/runpy.py", line 95, in run_module
    filename, loader, alter_sys)
  File "/usr/lib64/python2.5/runpy.py", line 52, in _run_module_code
    mod_name, mod_fname, mod_loader)
  File "/usr/lib64/python2.5/runpy.py", line 32, in _run_code
    exec code in run_globals
  File "/srv/pubPal/test/test_auth.py", line 16, in <module>
    print opener.open(uri)
  File "/usr/lib64/python2.5/urllib2.py", line 380, in open
    response = meth(req, response)
  File "/usr/lib64/python2.5/urllib2.py", line 491, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib64/python2.5/urllib2.py", line 412, in error
    result = self._call_chain(*args)
  File "/usr/lib64/python2.5/urllib2.py", line 353, in _call_chain
    result = func(*args)
  File "/usr/lib64/python2.5/urllib2.py", line 992, in http_error_401
    host, req, headers)
  File "/usr/lib64/python2.5/urllib2.py", line 884, in http_error_auth_reqed
    return self.retry_http_digest_auth(req, authreq)
  File "/usr/lib64/python2.5/urllib2.py", line 889, in
retry_http_digest_auth
    auth = self.get_authorization(req, chal)
  File "/usr/lib64/python2.5/urllib2.py", line 920, in get_authorization
    H, KD = self.get_algorithm_impls(algorithm)
  File "/usr/lib64/python2.5/urllib2.py", line 972, in get_algorithm_impls
    return H, KD
UnboundLocalError: local variable 'H' referenced before assignment


The offending code is urllib2.py line 972:

    def get_algorithm_impls(self, algorithm):
        # lambdas assume digest modules are imported at the top level
        if algorithm == 'MD5':
            H = lambda x: hashlib.md5(x).hexdigest()
        elif algorithm == 'SHA':
            H = lambda x: hashlib.sha1(x).hexdigest()
        # XXX MD5-sess
        KD = lambda s, d: H("%s:%s" % (s, d))
        return H, KD

I'm not sure what's meant by the # XXX MD5-sess comment there... But
what ends up happening is that MD5-sess matches neither the if or the
elif, and when return attempts to return, H is undefined.

This could be easily patched, but support for MD5-sess would be great as
well.  

In my estimation, at the least, urllib2 shouldn't crap out in such an
obscure way when encountering an unsupported algorithm.

I'd suggest changing line 970 (The # XXX MD5-sess line) to:

 raise Exception, "Unsupported Digest Authentication Algorithm '%s'" %
(algorithm)