On the page lib/http-redirect-handler.html it says the signature of 
redirect_request is:

redirect_request(  req, fp, code, msg, hdrs)

It is actually:

redirect_request(req, fp, code, msg, hdrs, newurl)

Well, technically the signature is:

redirect_request(self, req, fp, code, msg, hdrs, newurl)

but it is called as the six-argument version.