How do I write a python HTTP server to listen on multiple ports?

Sure; just start two different servers on two different ports in two different threads that each use the same handler. Here's a complete, working example that I just wrote and tested. If you run this code then you'll be able to get a Hello World webpage at both http://localhost:1111/ and http://localhost:2222/

from threading import Thread
from SocketServer import ThreadingMixIn
from BaseHTTPServer import HTTPServer, BaseHTTPRequestHandler

class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-type", "text/plain")
        self.end_headers()
        self.wfile.write("Hello World!")

class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
    daemon_threads = True

def serve_on_port(port):
    server = ThreadingHTTPServer(("localhost",port), Handler)
    server.serve_forever()

Thread(target=serve_on_port, args=[1111]).start()
serve_on_port(2222)

update:

This also works with Python 3 but three lines need to be slightly changed:

from socketserver import ThreadingMixIn
from http.server import HTTPServer, BaseHTTPRequestHandler

and

self.wfile.write(bytes("Hello World!", "utf-8"))

Not easily. You could have two ThreadingHTTPServer instances, write your own serve_forever() function (don't worry it's not a complicated function).

The existing function:

def serve_forever(self, poll_interval=0.5):
    """Handle one request at a time until shutdown.

    Polls for shutdown every poll_interval seconds. Ignores
    self.timeout. If you need to do periodic tasks, do them in
    another thread.
    """
    self.__serving = True
    self.__is_shut_down.clear()
    while self.__serving:
        # XXX: Consider using another file descriptor or
        # connecting to the socket to wake this up instead of
        # polling. Polling reduces our responsiveness to a
        # shutdown request and wastes cpu at all other times.
        r, w, e = select.select([self], [], [], poll_interval)
        if r:
            self._handle_request_noblock()
    self.__is_shut_down.set()

So our replacement would be something like:

def serve_forever(server1,server2):
    while True:
        r,w,e = select.select([server1,server2],[],[],0)
        if server1 in r:
            server1.handle_request()
        if server2 in r:
            server2.handle_request()