Overriding urllib2.HTTPError or urllib.error.HTTPError and reading response HTML anyway

The HTTPError is a file-like object. You can catch it and then read its contents.

try:
    resp = urllib2.urlopen(url)
    contents = resp.read()
except urllib2.HTTPError, error:
    contents = error.read()

If you mean you want to read the body of the 500:

request = urllib2.Request(url, data, headers)
try:
        resp = urllib2.urlopen(request)
        print resp.read()
except urllib2.HTTPError, error:
        print "ERROR: ", error.read()

In your case, you don't need to build up the request. Just do

try:
        resp = urllib2.urlopen(url)
        print resp.read()
except urllib2.HTTPError, error:
        print "ERROR: ", error.read()

so, you don't override urllib2.HTTPError, you just handle the exception.