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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.

Quick links

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()
    app.listen(8888)
    tornado.ioloop.IOLoop.current().start()

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

Threads and WSGI

Tornado is different from most Python web frameworks. It is not based on WSGI, and it is typically run with only one thread per process. See the :doc:`guide` for more on Tornado's approach to asynchronous programming.

While some support of WSGI is available in the tornado.wsgi module, it is not a focus of development and most applications should be written to use Tornado's own interfaces (such as tornado.web) directly instead of using WSGI.

In general, Tornado code is not thread-safe. The only method in Tornado that is safe to call from other threads is .IOLoop.add_callback. You can also use .IOLoop.run_in_executor to asynchronously run a blocking function on another thread, but note that the function passed to run_in_executor should avoid referencing any Tornado objects. run_in_executor is the recommended way to interact with blocking code.

Installation

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 5.x runs on Python 2.7, and 3.4+ (Tornado 6.0 will require Python 3.5+; Python 2 will no longer be supported). The updates to the ssl module in Python 2.7.9 are required (in some distributions, these updates may be available in older python versions). In addition to the requirements which will be installed automatically by pip or setup.py install, the following optional packages may be useful:

  • 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 adjustments are frequent. No longer needed in Python 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.

Documentation

This documentation is also available in PDF and Epub formats.

.. toctree::
   :titlesonly:

   guide
   webframework
   http
   networking
   coroutine
   integration
   utilities
   faq
   releases

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.