Sorry, went a bit too quickly -- here is the sample code that I meant to use:

  import os, urllib2
  os.environ['http_proxy'] = "http://someuser:a/b@10.11.12.13:1234"
  f = urllib2.urlopen('http://www.python.org')
  data = f.read()
  print data

And the stack trace that I receive:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    f = urllib2.urlopen('http://www.python.org')
  File "/usr/lib64/python2.7/urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib64/python2.7/urllib2.py", line 431, in open
    response = self._open(req, data)
  File "/usr/lib64/python2.7/urllib2.py", line 449, in _open
    '_open', req)
  File "/usr/lib64/python2.7/urllib2.py", line 409, in _call_chain
    result = func(*args)
  File "/usr/lib64/python2.7/urllib2.py", line 1227, in http_open
    return self.do_open(httplib.HTTPConnection, req)
  File "/usr/lib64/python2.7/urllib2.py", line 1166, in do_open
    h = http_class(host, timeout=req.timeout, **http_conn_args)
  File "/usr/lib64/python2.7/httplib.py", line 712, in __init__
    (self.host, self.port) = self._get_hostport(host, port)
  File "/usr/lib64/python2.7/httplib.py", line 754, in _get_hostport
    raise InvalidURL("nonnumeric port: '%s'" % host[i+1:])
httplib.InvalidURL: nonnumeric port: 'a'

It actually looks the same -- so I suppose this issue affects both urllib and urllib2.