In fact, urllib.proxy_bypass() returns True for any simple host name (i.e. any string without '.' in it):

>>> proxy_bypass('xyz')
True
>>> proxy_bypass('')
True
>>> proxy_bypass('whatever')
True

I think the fix I am proposing makes sense regardless of platform because proxy_url is set up numerically:

proxy_url = "http://127.0.0.1:%d" % self.server.port

Maybe the above should be changed to

proxy_url = "%s:%d" % (self.URL, self.server.port)

once URL is changed to numerical form.