.. title:: Tornado Web Server

.. meta::
    :google-site-verification: g4bVhgwbVO1d9apCUsT-eKlApg31Cygbp8VGZY8Rf0g

|Tornado Web Server|
====================

.. |Tornado Web Server| image:: tornado.png
    :alt: Tornado Web Server

`Tornado <http://www.tornadoweb.org>`_ is a Python web framework and
asynchronous networking library, originally developed at `FriendFeed
<http://friendfeed.com>`_.  By using non-blocking network I/O, Tornado
can scale to tens of thousands of open connections, making it ideal for
`long polling <http://en.wikipedia.org/wiki/Push_technology#Long_polling>`_,
`WebSockets <http://en.wikipedia.org/wiki/WebSocket>`_, and other
applications that require a long-lived connection to each user.

Quick links
-----------

* Current version: |version| (`download from PyPI <https://pypi.python.org/pypi/tornado>`_, :doc:`release notes <releases>`)
* `Source (GitHub) <https://github.com/tornadoweb/tornado>`_
* Mailing lists: `discussion <http://groups.google.com/group/python-tornado>`_ and `announcements <http://groups.google.com/group/python-tornado-announce>`_
* `Stack Overflow <http://stackoverflow.com/questions/tagged/tornado>`_
* `Wiki <https://github.com/tornadoweb/tornado/wiki/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
<https://github.com/tornadoweb/tornado/tree/stable/demos/chat>`_.

Threads and WSGI
----------------

Tornado is different from most Python web frameworks. It is not based
on `WSGI <https://wsgi.readthedocs.io/en/latest/>`_, 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.

``asyncio`` Integration
-----------------------

Tornado is integrated with the standard library `asyncio` module and
shares the same event loop (by default since Tornado 5.0). In general,
libraries designed for use with `asyncio` can be mixed freely with
Tornado.


Installation
------------

::

    pip install tornado

Tornado is listed in `PyPI <http://pypi.python.org/pypi/tornado>`_ 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
<https://github.com/tornadoweb/tornado>`_ as well.

**Prerequisites**: Tornado 6.0 requires Python 3.5.2 or newer (See
`Tornado 5.1 <https://www.tornadoweb.org/en/branch5.1/>`_ if
compatibility with Python 2.7 is required). The following optional
packages may be useful:

* `pycurl <http://pycurl.sourceforge.net>`_ is used by the optional
  ``tornado.curl_httpclient``.  Libcurl version 7.22 or higher is required.
* `Twisted <http://www.twistedmatrix.com>`_ may be used with the classes in
  `tornado.platform.twisted`.
* `pycares <https://pypi.python.org/pypi/pycares>`_ is an alternative
  non-blocking DNS resolver that can be used when threads are not
  appropriate.

**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.

On Windows, Tornado requires the ``WindowsSelectorEventLoop``. This is
the default in Python 3.7 and older, but Python 3.8 defaults to an
event loop that is not compatible with Tornado. Applications that use
Tornado on Windows with Python 3.8 must call
``asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())``
at the beginning of their ``main`` file/function.

Documentation
-------------

This documentation is also available in `PDF and Epub formats
<https://readthedocs.org/projects/tornado/downloads/>`_.

.. toctree::
   :titlesonly:

   guide
   webframework
   http
   networking
   coroutine
   integration
   utilities
   faq
   releases

* :ref:`genindex`
* :ref:`modindex`
* :ref:`search`

Discussion and support
----------------------

You can discuss Tornado on `the Tornado developer mailing list
<http://groups.google.com/group/python-tornado>`_, and report bugs on
the `GitHub issue tracker
<https://github.com/tornadoweb/tornado/issues>`_.  Links to additional
resources can be found on the `Tornado wiki
<https://github.com/tornadoweb/tornado/wiki/Links>`_.  New releases are
announced on the `announcements mailing list
<http://groups.google.com/group/python-tornado-announce>`_.

Tornado is available under
the `Apache License, Version 2.0
<http://www.apache.org/licenses/LICENSE-2.0.html>`_.

This web site and all documentation is licensed under `Creative
Commons 3.0 <http://creativecommons.org/licenses/by/3.0/>`_.
