Squid-style external redirector for polipo doesn't work

I am trying to write a trivial squid-style redirector for polipo http proxy, as specified in the documentation.

Here is the code:

#!/usr/bin/python

# based on
# http://gofedora.com/how-to-write-custom-redirector-rewritor-plugin-squid-python/

import sys

def modify_url(line):
    l = line.split(" ")
    old_u开发者_如何学Pythonrl = l[0]
    new_url = "\n"
    if "experts-exchange" in old_url:
        new_url = "http://127.0.0.1/" + new_url
    return new_url

while True:
    line = sys.stdin.readline().strip()
    new_url = modify_url(line)
    sys.stdout.write(new_url)
    sys.stdout.flush()

When running polipo with this redirector and trying to access http://www.experts-exchange.com/, I get the following error:

500 Couldn't test for forbidden URL: Redirector error

Actually, I get the same error when trying to access any URL, which makes me think it's a problem with my redirector code.

The output in polipo's log doesn't provide more hints, all I see there is:

Redirector returned incomplete reply.

What am I doing wrong?

Edit: I have fixed modify_url() to return a value, because it did not. I'm still getting the same error.

That site (gofedora) mentions that you need to return either a blank line or the modified url in modify_url to make it work. Also you'll see the error if you run this manually. Your solution is to return new_url at the end of modify_url.

Please note that you also need to chmod +x to permit your proxy to run the script.