Doc/howto/webservers.rst

*******************************
  HOWTO Use Python in the web
*******************************

:Author: Marek Kubica

.. topic:: Abstract

   This document shows how Python fits into the web.  It presents some ways on
   how to integrate Python with the web server and general practices useful for
   developing web sites.


Programming for the Web has become a hot topic since the raise of the "Web 2.0",
which focuses on user-generated content on web sites.  It has always been
possible to use Python for creating web sites, but it was a rather tedious task.
Therefore, many so-called "frameworks" and helper tools were created to help
developers creating sites faster and these sites being more robust.  This HOWTO
describes some of the methods used to combine Python with a web server to create
dynamic content.  It is not meant as a general introduction as this topic is far
too broad to be covered in one single document.  However, a short overview of
the most popular libraries is provided.

.. seealso::

   While this HOWTO tries to give an overview over Python in the Web, it cannot
   always be as up to date as desired.  Web development in Python is moving
   forward rapidly, so the wiki page on `Web Programming
   <http://wiki.python.org/moin/WebProgramming>`_ might be more in sync with
   recent development.


The low-level view
==================

.. .. image:: http.png

When a user enters a web site, his browser makes a connection to the site's
webserver (this is called the *request*).  The server looks up the file in the
file system and sends it back to the user's browser, which displays it (this is
the *response*).  This is roughly how the unterlying protocol, HTTP works.

Now, dynamic web sites are not files in the file system, but rather programs
which are run by the web server when a request comes in.  They can do all sorts
of useful things, like display the postings of a bulletin board, show your
mails, configurate software or just display the current time.  These programs
can be written in about any programming language the server supports, so it is
easy to use Python for creating dynamic web sites.

As most of HTTP servers are written in C or C++, they cannot execute Python code
in a simple way -- a bridge is needed between the server and the program.  These
bridges or rather interfaces define how programs interact with the server.  In
the past there have been numerous attempts to create the best possible
interface, but there are only a few worth mentioning.

Not every web server supports every interface.  Many web servers do support only
old, now-obsolete interfaces.  But they can often be extended using some
third-party modules to support new interfaces.


Common Gateway Interface
------------------------

This interface is the oldest one, supported by nearly every web server out of
the box.  Programs using CGI to communicate with their web server need to be
started by the server for every request.  So, every request starts a new Python
interpreter -- which takes some time to start up -- thus making the whole
interface only usable for low load situations.

The upside of CGI is that it is simple -- writing a program which uses CGI is a
matter of about three lines of code.  But this simplicity comes at a price: it
does very few things to help the developer.

Writing CGI programs, while still possible, is not recommended anymore.  With
WSGI (more on that later) it is possible to write programs that emulate CGI, so
they can be run as CGI if no better option is available.

.. seealso::

   The Python standard library includes some modules that are helpful for
   creating plain CGI programs:

   * :mod:`cgi` -- Handling of user input in CGI scripts
   * :mod:`cgitb` -- Displays nice tracebacks when errors happen in of CGI
     applications, instead of presenting a "500 Internal Server Error" message

   The Python wiki features a page on `CGI scripts
   <http://wiki.python.org/moin/CgiScripts>`_ with some additional information
   about CGI in Python.

   
Simple script for testing CGI
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To test whether your web server works with CGI, you can use this short and
simple CGI program::

    #!/usr/bin/env python
    # -*- coding: UTF-8 -*-

    # enable debugging
    import cgitb; cgitb.enable()

    print "Content-Type: text/plain;charset=utf-8"
    print

    print "Hello World!"

You need to write this code into a file with a ``.py`` or ``.cgi`` extension,
this depends on your web server configuration.  Depending on your web server
configuration, this file may also need to be in a ``cgi-bin`` folder, for
security reasons.

You might wonder what the ``cgitb`` line is about.  This line makes it possible
to display a nice traceback instead of just crashing and displaying an "Internal
Server Error" in the user's browser.  This is useful for debugging, but it might
risk exposing some confident data to the user.  Don't use it when the script is
ready for production use.  Still, you should *always* catch exceptions, and
display proper error pages -- end-users don't like to see nondescript "Internal
Server Errors" in their browsers.


Setting up CGI on your own server
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you don't have your own web server, this does not apply to you.  You can
check whether if works as-is and if not you need to talk to the administrator of
your web server anyway. If it is a big hoster, you can try filing a ticket
asking for Python support.

If you're your own administrator or want to install it for testing purposes on
your own computers, you have to configure it by yourself.  There is no one and
single way on how to configure CGI, as there are many web servers with different
configuration options.  The currently most widely used free web server is
`Apache HTTPd <http://httpd.apache.org/>`_, Apache for short -- this is the one
that most people use, it can be easily installed on nearly every system using
the systems' package management.  But `lighttpd <http://www.lighttpd.net>`_ has
been gaining attention since some time and is said to have a better performance.
On many systems this server can also be installed using the package management,
so manually compiling the web server is never needed.

* On Apache you can take a look into the `Dynamic Content with CGI
  <http://httpd.apache.org/docs/2.2/howto/cgi.html>`_ tutorial, where everything
  is described.  Most of the time it is enough just to set ``+ExecCGI``.  The
  tutorial also describes the most common gotchas that might arise.
* On lighttpd you need to use the `CGI module
  <http://trac.lighttpd.net/trac/wiki/Docs%3AModCGI>`_ which can be configured
  in a straightforward way.  It boils down to setting ``cgi.assign`` properly.


Common problems with CGI scripts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Trying to use CGI sometimes leads to small annoyances that one might experience
while trying to get these scripts to run.  Sometimes it happens that a seemingly
correct script does not work as expected, which is caused by some small hidden
reason that's difficult to spot.

Some of these reasons are:

* The Python script is not marked executable.  When CGI scripts are not
  executable most of the web servers will let the user download it, instead of
  running it and sending the output to the user.  For CGI scripts to run
  properly the ``+x`` bit needs to be set.  Using ``chmod a+x your_script.py``
  might already solve the problem.
* The line endings must be of Unix-type.  This is important because the web
  server checks the first line of the script (called shebang) and tries to run
  the program specified there.  It gets easily confused by Windows line endings
  (Carriage Return & Line Feed, also called CRLF), so you have to convert the
  file to Unix line endings (only Line Feed, LF).  This can be done
  automatically by uploading the file via FTP in text mode instead of binary
  mode, but the preferred way is just telling your editor to save the files with
  Unix line endings.  Most proper editors support this.
* Your web server must be able to read the file, you need to make sure the
  permissions are fine.  Often the server runs as user and group ``www-data``,
  so it might be worth a try to change the file ownership or making the file
  world readable by using ``chmod a+r your_script.py``.
* The webserver must be able to know that the file you're trying to access is a
  CGI script.  Check the configuration of your web server, maybe there is some
  mistake.
* The path to the interpreter in the shebang (``#!/usr/bin/env python``) must be
  currect.  This line calls ``/usr/bin/env`` to find Python, but it'll fail if
  there is no ``/usr/bin/env``.  If you know where your Python is installed, you
  can also use that path.  The commands ``whereis python`` and ``type -p
  python`` might also help to find where it is installed.  Once this is known,
  the shebang line can be changed accordingly: ``#!/usr/bin/python``.
* The file must not contain a BOM (Byte Order Mark). The BOM is meant for
  determining the byte order of UTF-16 encodings, but some editors write this
  also into UTF-8 files.  The BOM interferes with the shebang line, so be sure
  to tell your editor not to write the BOM.
* :ref:`mod-python` might be making problems.  mod_python is able to handle CGI
  scripts by itself, but it can also be a source for problems.  Be sure you
  disable it.


.. _mod-python:

mod_python
----------

People coming from PHP often find it hard to grasp how to use Python in the web.
Their first thought is mostly `mod_python <http://www.modpython.org/>`_ because
they think that this is the equivalent to ``mod_php``.  Actually it is not
really.  It does embed the interpreter into the Apache process, thus speeding up
requests by not having to start a Python interpreter every request.  On the
other hand, it is by far not "Python intermixed with HTML" as PHP often does.
The Python equivalent of that is a template engine.  mod_python itself is much
more powerful and gives more access to Apache internals.  It can emulate CGI, it
can work an a "Python Server Pages" mode similar to JSP which is "HTML
intermangled with Python" and it has a "Publisher" which destignates one file to
accept all requests and decide on what to do then.

But mod_python has some problems.  Unlike the PHP interpreter the Python
interpreter uses caching when executing files, so when changing a file the whole
web server needs to be re-started to update.  Another problem ist the basic
concept -- Apache starts some child processes to handle the requests and
unfortunately every child process needs to load the whole Python interpreter
even if it does not use it.  This makes the whole web server slower.  Another
problem is that as mod_python is linked against a specific version of
``libpython``, it is not possible to switch from an older version to a newer
(e.g. 2.4 to 2.5) without recompiling mod_python.  mod_python is also bound to
the Apache web server, so programs written for mod_python cannot easily run on
other web servers.

These are the reasons why mod_python should be avoided when writing new
programs.  In some circumstances it might be still a good idea to use mod_python
for deployment, but WSGI makes it possible to run WSGI programs under mod_python
as well.


FastCGI and SCGI
----------------

FastCGI and SCGI try to solve the performance problem of CGI in another way.
Instead of embedding the interpreter into the web server, they create
long-running processes which run in the background. There still is some module
in the web server which makes it possible for the web server to "speak" with the
background process.  As the background process is independent from the server,
it can be written in any language of course also in Python.  The language just
needs to have a library which handles the communication with the web server.

The difference between FastCGI and SCGI is very small, as SCGI is essentially
just a "simpler FastCGI".  But as the web server support for SCGI is limited
most people use FastCGI instead, which works the same way.  Almost everything
that applies to SCGI also applies to FastCGI as well, so we'll only write about
the latter.

These days, FastCGI is never used directly.  Just like ``mod_python`` it is only
used for the deployment of WSGI applications.

.. seealso::

   * `FastCGI, SCGI, and Apache: Background and Future
     <http://www.vmunix.com/mark/blog/archives/2006/01/02/fastcgi-scgi-and-apache-background-and-future/>`_
     is a discussion on why the concept of FastCGI and SCGI is better that that
     of mod_python.


Setting up FastCGI
^^^^^^^^^^^^^^^^^^

Depending on the web server you need to have a special module.

* Apache has both `mod_fastcgi <http://www.fastcgi.com/>`_ and `mod_fcgid
  <http://fastcgi.coremail.cn/>`_.  ``mod_fastcgi`` is the original one, but it
  has some licensing issues that's why it is sometimes considered non-free.
  ``mod_fcgid`` is a smaller, compatible alternative. One of these modules needs
  to be loaded by Apache.
* lighttpd ships its own `FastCGI module
  <http://trac.lighttpd.net/trac/wiki/Docs%3AModFastCGI>`_ as well as an `SCGI
  module <http://trac.lighttpd.net/trac/wiki/Docs%3AModSCGI>`_.
* nginx also supports `FastCGI
  <http://wiki.codemongers.com/NginxSimplePythonFCGI>`_.

Once you have installed and configured the module, you can test it with the
following WSGI-application::

    #!/usr/bin/env python
    # -*- coding: UTF-8 -*-

    from cgi import escape
    import sys, os
    from flup.server.fcgi import WSGIServer

    def app(environ, start_response):
        start_response('200 OK', [('Content-Type', 'text/html')])

        yield '<h1>FastCGI Environment</h1>'
        yield '<table>'
        for k, v in sorted(environ.items()):
             yield '<tr><th>%s</th><td>%s</td></tr>' % (escape(k), escape(v))
        yield '</table>'

    WSGIServer(app).run()

This is a simple WSGI application, but you need to install `flup
<http://pypi.python.org/pypi/flup/1.0>`_ first, as flup handles the low level
FastCGI access.

.. seealso::

   There is some documentation on `setting up Django with FastCGI
   <http://www.djangoproject.com/documentation/fastcgi/>`_, most of which can be
   reused for other WSGI-compliant frameworks and libraries.  Only the
   ``manage.py`` part has to be changed, the example used here can be used
   instead. Django does more or less the exact same thing.


mod_wsgi
--------

`mod_wsgi <http://www.modwsgi.org/>`_ is an attempt to get rid of the low level
gateways.  As FastCGI, SCGI, mod_python are mostly used to deploy WSGI
applications anyway, mod_wsgi was started to directly embed WSGI aplications
into the Apache web server.  The benefit from this approach is that WSGI
applications can be deployed much easier as is is specially designed to host
WSGI applications -- unlike the other low level methods which have glue code to
host WSGI applications (like flup which was mentioned before).  The downside is
that mod_wsgi is limited to the Apache web server, other servers would need
their own implementations of mod_wsgi.

It supports two modes: the embedded mode in which it integrates with the Apache
process and the daemon mode which is more FastCGI-like.  Contrary to FastCGI,
mod_wsgi handles the worker-processes by itself which makes administration
easier.


.. _WSGI:

Step back: WSGI
===============

WSGI was already mentioned several times so it has to be something important.
In fact it really is, so now it's time to explain.

The *Web Server Gateway Interface*, :pep:`333` or WSGI for short is currently
the best possible way to Python web programming.  While it is great for
programmers writing frameworks, the normal person does not need to get in direct
contact with it.  But when choosing a framework for web development it is a good
idea to take one which supports WSGI.

The big profit from WSGI is the unification.  When your program is compatible
with WSGI -- that means that your framework has support for WSGI, your program
can be deployed on every web server interface for which there are WSGI wrappers.
So you do not need to care about whether the user uses mod_python or FastCGI --
with WSGI it just works on any gateway interface.  The Python standard library
contains its own WSGI server :mod:`wsgiref`, which is a small web server that
can be used for testing.

A really great WSGI feature are the middlewares.  Middlewares are layers around
your program which can add various functionality to it.  There is a `number of
middlewares <http://wsgi.org/wsgi/Middleware_and_Utilities>`_ already available.
For example, instead of writing your own session management (to identify a user
in subsequent requests, as HTTP does not maintain state, so it does now know
that the requests belong to the same user) you can just take one middleware,
plug it in and you can rely an already existing functionality.  The same thing
is compression -- say you want to compress your HTML using gzip, to save your
server's bandwidth.  So you only need to plug-in a middleware and you're done.
Authentication is also a problem easily solved using a middleware.

So, generally -- although WSGI may seem complex, the initial phase of learning
can be very rewarding as WSGI does already have solutions to many problems that
might arise while writing web sites.


WSGI Servers
------------

The code that is used to connect to various low level gateways like CGI or
mod_python is called *WSGI server*.  One of these servers is ``flup`` which was
already mentioned and supports FastCGI, SCGI as well as `AJP
<http://en.wikipedia.org/wiki/Apache_JServ_Protocol>`_.  Some of these servers
are written in Python as ``flup`` is, but there also exist others which are
written in C and can be used as drop-in replacements.

There are quite a lot of servers already available, so a Python web application
can be deployed nearly everywhere.  This is one big advantage that Python has
compared with other web techniques.

.. seealso::

   A good overview of all WSGI-related code can be found in the `WSGI wiki
   <http://wsgi.org/wsgi>`_, which contains an extensive list of `WSGI servers
   <http://wsgi.org/wsgi/Servers>`_, which can be used by *every* application
   supporting WSGI.

   You might be interested in some WSGI-supporting modules already contained in
   the standard library, namely:
    
   * :mod:`wsgiref` -- some tiny utilities and servers for WSGI


Case study: MoinMoin
--------------------

What does WSGI give the web application developer?  Let's take a look on one
long existing web application written in Python without using WSGI.

One of the most widely used wiki software is `MoinMoin <http://moinmo.in/>`_.
It was created in 2000, so it predates WSGI by about three years.  While it now
includes support for WSGI, older versions needed separate code to run on CGI,
mod_python, FastCGI and standalone.  Now, this all is possible by using WSGI and
the already-written gateways.  For running with on FastCGI ``flup`` can be used,
for running a standalone server :mod:`wsgiref` is the way to go.


Model-view-controller
=====================

The term *MVC* is often heard in statements like "framework *foo* supports MVC".
While MVC is not really something technical but rather organisational, many web
frameworks use this model to help the developer to bring structure into his
program.  Bigger web applications can have lots of code so it is a good idea to
have structure in the program right from the beginnings.  That way, even users
of other frameworks (or even languages, as MVC is nothing Python-specific) can
understand the existing code easier, as they are already familiar with the
structure.

MVC stands for three components:

* The *model*.  This is the data that is meant to modify.  In Python frameworks
  this component is often represented by the classes used by the
  object-relational mapper.  So, all declarations go here.
* The *view*.  This component's job is to display the data of the model to the
  user.  Typically this component is represented by the templates.
* The *controller*.  This is the layer between the user and the model.  The
  controller reacts on user actions (like opening some specific URL) and tells
  the model to modify the data if neccessary.

While one might think that MVC is a complex design pattern, in fact it is not.
It is used in Python because it has turned out to be useful for creating clean,
maintainable web sites.

.. note::

   While not all Python frameworks explicitly support MVC, it is often trivial
   to create a web site which uses the MVC pattern by seperating the data logic
   (the model) from the user interaction logic (the controller) and the
   templates (the view).  That's why it is important not to write unneccessary
   Python code in the templates -- it is against MVC and creates more chaos.

.. seealso::

   The english Wikipedia has an article about the `Model-View-Controller pattern
   <http://en.wikipedia.org/wiki/Model-view-controller>`_, which includes a long
   list of web frameworks for different programming languages.


Ingredients for web sites
=========================

Web sites are complex constructs, so tools were created to help the web site
developer to make his work maintainable.  None of these tools are in any way
Python specific, they also exist for other programming languages as well.  Of
course, developers are not forced to use these tools and often there is no
"best" tool, but it is worth informing yourself before choosing something
because of the big number of helpers that the developer can use.


.. seealso::

   People have written far more components that can be combined than these
   presented here.  The Python wiki has a page about these components, called
   `Web Components <http://wiki.python.org/moin/WebComponents>`_.


Templates
---------

Mixing of HTML and Python code is possible with some libraries.  While
convenient at first, it leads to horribly unmaintainable code.  That's why
templates exist.  Templates are, in the simplest case, just HTML files with
placeholders.  The HTML is sent to the user's browser after filling out the
placeholders.

Python already includes such simple templates::

    # a simple template
    template = "<html><body><h1>Hello %s!</h1></body></html>"
    print template % "Reader"

The Python standard library also includes some more advanced templates usable
through :class:`string.Template`, but in HTML templates it is needed to use
conditional and looping contructs like Python's *for* and *if*.  So, some
*template engine* is needed.

Now, Python has a lot of template engines which can be used with or without a
`framework`_.  Some of these are using a plain-text programming language which
is very easy to learn as it is quite limited while others use XML so the
template output is always guaranteed to be valid XML.  Some `frameworks`_ ship
their own template engine or recommend one particular.  If one is not yet sure,
using these is a good idea.

.. note::

   While Python has quite a lot of different template engines it usually does
   not make sense to use a homebrewed template system.  The time needed to
   evaluate all templating systems is not really worth it, better invest the
   time in looking through the most popular ones.  Some frameworks have their
   own template engine or have a recommentation for one.  It's wise to use
   these.
  
   Popular template engines include:

   * Mako
   * Genshi
   * Jinja

.. seealso::

   Lots of different template engines divide the attention between themselves
   because it's easy to create them in Python.  The page `Templating
   <http://wiki.python.org/moin/Templating>`_ in the wiki lists a big,
   ever-growing number of these.


Data persistence
----------------

*Data persistence*, while sounding very complicated is just about storing data.
This data might be the text of blog entries, the postings of a bulletin board or
the text of a wiki page.  As always, there are different ways to store
informations on a web server.

Often relational database engines like `MySQL <http://www.mysql.com/>`_ or
`PostgreSQL <http://http://www.postgresql.org/>`_ are used due to their good
performance handling very large databases consisting of up to millions of
entries.  These are *queried* using a language called `SQL
<http://en.wikipedia.org/wiki/SQL>`_.  Python programmers in general do not like
SQL too much, they prefer to work with objects.  It is possible to save Python
objects into a database using a technology called `ORM
<http://en.wikipedia.org/wiki/Object-relational_mapping>`_.  ORM translates all
object-oriented access into SQL code under the hood, the user does not need to
think about it.  Most `frameworks`_ use ORMs and it works quite well.

A second possibility is using files that are saved on the hard disk (sometimes
called flatfiles).  This is very easy, but is not too fast.  There is even a
small database engine called `SQLite <http://www.sqlite.org/>`_ which is bundled
with Python in the :mod:`sqlite` module and uses only one file.  This database
can be used to store objects via an ORM and has no other dependencies.  For
smaller sites SQLite is just enough.  But it is not the only way in which data
can be saved into the file systems.  Sometimes normal, plain text files are
enough.

The third and least used possibility are so-called object oriented databases.
These databases store the *actual objects* instead of the relations that
OR-mapping creates between rows in a database.  This has the advantage that
nearly all objects can be saven in a straightforward way, unlike in relational
databases where some objects are very hard to represent with ORMs.

`Frameworks`_ often give the users hints on which method to choose, it is
usually a good idea to stick to these unless there are some special requirements
which require to use the one method and not the other.

.. seealso::

   * `Persistence Tools <http://wiki.python.org/moin/PersistenceTools>`_ lists
     possibilities on how to save data in the file system, some of these modules
     are part of the standard library
   * `Database Programming <http://wiki.python.org/moin/DatabaseProgramming>`_
     helps on choosing a method on how to save the data
   * `SQLAlchemy <http://www.sqlalchemy.org/>`_, the most powerful OR-Mapper for
     Python and `Elixir <http://elixir.ematia.de/>`_ which makes it easier to
     use
   * `SQLObject <http://www.sqlobject.org/>`_, another popular OR-Mapper
   * `ZODB <https://launchpad.net/zodb>`_ and `Durus
     <http://www.mems-exchange.org/software/durus/>`_, two object oriented
     databases


.. _framework:

Frameworks
==========

As web sites can easily become quite large, there are so-called frameworks which
were created to help the developer with making these sites.  Although the most
well-known framework is Ruby on Rails, Python does also have its own frameworks
which are partly inspired by Rails or which were existing a long time before
Rails.

Two possible approaches to web frameworks exist: the minimalistic approach and
the all-inclusive approach (somtimes called *full-stack*). Frameworks which are
all-inclusive give you everything you need to start working, like a template
engine, some way to save and access data in databases and many features more.
Most users are best off using these as they are widely used by lots of other
users and well documented in form of books and tutorials.  Other web frameworks
go the minimalistic approach trying to be as flexible as possible leaving the
user the freedom to choose what's best for him.

The majority of users is best off with all-inclusive framewors.  They bring
everything along so a user can just jump in and start to code.  While they do
have some limitations they can fullfill 80% of what one will ever want to
perfectly.  They consist of various components which are designed to work
together as good as possible.

The multitude of web frameworks written in Python demonstrates that it is really
easy to write one.  One of the most well-known web applications written in
Python is `Zope <http://www.zope.org/>`_ which can be regarded as some kind of
big framework.  But Zope was not the only framework, there were some others
which are by now nearly forgotten.  These do not need to be mentioned anymore,
because most people that used them moved on to newer ones.


Some notable frameworks
-----------------------

There is an incredible number of frameworks, so there is no way to describe them
all.  It is not even neccessary, as most of these frameworks are nothing special
and everything that can be done with these can also be done with one of the
popular ones.


Django
^^^^^^

`Django <http://www.djangoproject.com/>`_ is a framework consisting of several
tightly coupled elements which were written from scratch and work together very
well.  It includes an ORM which is quite powerful while being simple to use and
has a great online administration interface which makes it possible to edit the
data in the database with a browser.  The template engine is text-based and is
designed to be usable for page designers who cannot write Python.  It supports
so-called template inheritance and filters (which work like Unix pipes).  Django
has many handy features bundled, like creation of RSS feeds or generic views
which make it possible to write web sites nearly without any Python code.

It has a big, international community which has created many sites using Django.
There are also quite a lot of add-on projects which extend Django's normal
functionality.  This is partly due to Django's well written `online
documentation <http://doc.djangoproject.com/>`_ and the `Django book
<http://www.djangobook.com/>`_.


.. note::

   Although Django is an MVC-style framework, it calls the components
   differently, which is described in the `Django FAQ
   <http://www.djangoproject.com/documentation/faq/#django-appears-to-be-a-mvc-framework-but-you-call-the-controller-the-view-and-the-view-the-template-how-come-you-don-t-use-the-standard-names>`_.


TurboGears
^^^^^^^^^^

The other popular web framework in Python is `TurboGears
<http://www.turbogears.org/>`_.  It takes the approach of using already existing
components and combining them with glue code to create a seamless experience.
TurboGears gives the user more flexibility on which components to choose, the
ORM can be switched between some easy to use but limited and complex but very
powerful.  Same goes for the template engine.  One strong point about TurboGears
is that the components that it consists of can be used easily in other projects
without depending on TurboGears, for example the underlying web server CherryPy.

The documentation can be found in the `TurboGears wiki
<http://docs.turbogears.org/>`_, where links to screencasts can be found.
TurboGears has also an active user community which can respond to most related
questions.  There is also a `TurboGears book <http://turbogearsbook.com/>`_
published, which is a good starting point.

The plan for the next major version of TurboGears, version 2.0 is to switch to a
more flexible base provided by another very flexible web framework called
`Pylons <http://pylonshq.com/>`_.


Other notable frameworks
^^^^^^^^^^^^^^^^^^^^^^^^

These two are of course not the only frameworks that are available, there are
also some less-popular frameworks worth mentioning.

One of these is the already mentioned Zope, which has been around for quite a
long time.  With Zope 2.x having been known as rather un-pythonic, the newer
Zope 3.x tries to change that and therefore gets more acceptance from Python
programmers.  These efforts already showed results, there is a project which
connects Zope with WSGI called `Repoze <http://repoze.org/>`_ and another
project called `Grok <http://grok.zope.org/>`_ which makes it possible for
"normal" Python programmers use the very mature Zope components.

Another framework that's already been mentioned is `Pylons`_.  Pylons is much
like TurboGears with ab even stronger emphasis on flexibility, which is bought
at the cost of being more difficult to use.  Nearly every component can be
exchanged, which makes it neccessary to use the documentation of every single
component, because there are so many Pylons combinations possible that can
satisfy every requirement.  Pylons builds upon `Paste
<http://pythonpaste.org/>`_, an extensive set of tools which are handy for WSGI.

And that's still not everything.  The most up-to-date information can always be
found in the Python wiki.

.. seealso::
    
   The Python wiki contains an extensive list of `web frameworks
   <http://wiki.python.org/moin/WebFrameworks>`_.

   Most frameworks also have their own mailing lists and IRC channels, look out
   for these on the projects' websites.  There is also a general "Python in the
   Web" IRC channel on freenode called `#python.web
   <http://wiki.python.org/moin/PoundPythonWeb>`_.