Heroku上的Tornado服务器。错误R10(启动超时)

我正在尝试部署一个使用Tornado服务器用Python编写的Web应用程序。

在我的 app.py 我有

if __name__ == '__main__':
    server = tornado.httpserver.HTTPServer(Application())
    server.listen(4200, address='0.0.0.0')
    tornado.ioloop.IOLoop.instance().start()

然后我有一个程序文件 web: python app.py

当我检查日志时,我看到了错误

2018-08-08T02:20:54.117821+00:00 heroku[web.1]: Starting process with command `python app.py`
2018-08-08T02:20:57.000000+00:00 app[api]: Build succeeded
2018-08-08T02:21:54.891301+00:00 heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
2018-08-08T02:21:54.891301+00:00 heroku[web.1]: Stopping process with SIGKILL
2018-08-08T02:21:55.027586+00:00 heroku[web.1]: Process exited with status 137
2018-08-08T02:21:55.049407+00:00 heroku[web.1]: State changed from starting to crashed
2018-08-08T02:21:55.051776+00:00 heroku[web.1]: State changed from crashed to starting
2018-08-08T02:21:59.562357+00:00 heroku[web.1]: Starting process with command `python app.py`

我不太清楚自己在做什么,尤其是 Procfile 所以我猜问题就在那里。