Tornado Web Server

Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.

Hello, world

Here is a simple “Hello, world” example web app for Tornado:

import tornado.ioloop
import tornado.web

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello, world")

def make_app():
    return tornado.web.Application([
        (r"/", MainHandler),

if __name__ == "__main__":
    app = make_app()

This example does not use any of Tornado’s asynchronous features; for that see this simple chat room.


pip install tornado

Tornado is listed in PyPI and can be installed with pip. Note that the source distribution includes demo applications that are not present when Tornado is installed in this way, so you may wish to download a copy of the source tarball or clone the git repository as well.

Prerequisites: Tornado runs on Python 2.7, and 3.3+ For Python 2, version 2.7.9 or newer is strongly recommended for the improved SSL support. In addition to the requirements which will be installed automatically by pip or install, the following optional packages may be useful:

  • concurrent.futures is the recommended thread pool for use with Tornado and enables the use of ThreadedResolver. It is needed only on Python 2; Python 3 includes this package in the standard library.
  • pycurl is used by the optional tornado.curl_httpclient. Libcurl version 7.22 or higher is required.
  • Twisted may be used with the classes in tornado.platform.twisted.
  • pycares is an alternative non-blocking DNS resolver that can be used when threads are not appropriate.
  • monotonic or Monotime add support for a monotonic clock, which improves reliability in environments where clock adjustements are frequent. No longer needed in Python 3.3.

Platforms: Tornado should run on any Unix-like platform, although for the best performance and scalability only Linux (with epoll) and BSD (with kqueue) are recommended for production deployment (even though Mac OS X is derived from BSD and supports kqueue, its networking performance is generally poor so it is recommended only for development use). Tornado will also run on Windows, although this configuration is not officially supported and is recommended only for development use. Without reworking Tornado IOLoop interface, it’s not possible to add a native Tornado Windows IOLoop implementation or leverage Windows’ IOCP support from frameworks like AsyncIO or Twisted.


This documentation is also available in PDF and Epub formats.

Discussion and support

You can discuss Tornado on the Tornado developer mailing list, and report bugs on the GitHub issue tracker. Links to additional resources can be found on the Tornado wiki. New releases are announced on the announcements mailing list.

Tornado is available under the Apache License, Version 2.0.

This web site and all documentation is licensed under Creative Commons 3.0.