Starting tornado on a random free port

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One of the software I maintain for work is a GUI data browser that uses Tornado as a backend and a web browser as a front-end.

It is quite convenient to start the command and have the browser open automatically on the right URL. It's quite annoying to start the command and be told that the default port is already in use.

I've needed this trick quite often, also when writing unit tests, and it's time I note it down somewhere, so it's easier to find than going through Tornado's unittest code where I found it the first time.

This is how to start Tornado on a free random port:

from tornado.options import define, options
import tornado.netutil
import tornado.httpserver

define("web_port", type=int, default=None, help="listening port for web interface")

application = Application(self.db_url)

if options.web_port is None:
    sockets = tornado.netutil.bind_sockets(0, '127.0.0.1')
    self.web_port = sockets[0].getsockname()[:2][1]
    server = tornado.httpserver.HTTPServer(application)
    server.add_sockets(sockets)
else:
    server = tornado.httpserver.HTTPServer(application)
    server.listen(options.web_port)