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

Author:
   Marek Kubica


Abstract
^^^^^^^^

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

Programming for the Web has become a hot topic since the rise of “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 frameworks and helper tools
have been created to assist developers in creating faster and more
robust sites.  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 complete 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.

See also: While this HOWTO tries to give an overview of Python in
  the web, it cannot always be as up to date as desired.  Web
  development in Python is rapidly moving forward, so the wiki page on
  Web Programming may be more in sync with recent development.


The Low-Level View
==================

When a user enters a web site, their browser makes a connection to the
site’s web server (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
underlying protocol, HTTP, works.

Dynamic web sites are not based on files in the file system, but
rather on programs which are run by the web server when a request
comes in, and which *generate* the content that is returned to the
user.  They can do all sorts of useful things, like display the
postings of a bulletin board, show your email, configure software, or
just display the current time.  These programs can be written in any
programming language the server supports.  Since most servers support
Python, it is easy to use Python to create dynamic web sites.

Most HTTP servers are written in C or C++, so they cannot execute
Python code directly – a bridge is needed between the server and the
program.  These bridges, or rather interfaces, define how programs
interact with the server. 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 only
support old, now-obsolete interfaces; however, they can often be
extended using third-party modules to support newer ones.


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

This interface, most commonly referred to as “CGI”, is the oldest, and
is 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 Python program
which uses CGI is a matter of about three lines of code.  This
simplicity comes at a price: it does very few things to help the
developer.

Writing CGI programs, while still possible, is no longer recommended.
With WSGI, a topic covered later in this document, it is possible to
write programs that emulate CGI, so they can be run as CGI if no
better option is available.

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

  * "cgi" – Handling of user input in CGI scripts

  * "cgitb" – Displays nice tracebacks when errors happen in CGI
    applications, instead of presenting a “500 Internal Server Error”
    message

  The Python wiki features a page on CGI scripts 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!"

Depending on your web server configuration, you may need to save this
code with a ".py" or ".cgi" extension.  Additionally, 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 confidential
data to the user.  You should not use "cgitb" in production code for
this reason.  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 it works as-is, and if not you will need to talk
to the administrator of your web server. If it is a big host, you can
try filing a ticket asking for Python support.

If you are your own administrator or want to set up CGI for testing
purposes on your own computers, you have to configure it by yourself.
There is no single way to configure CGI, as there are many web servers
with different configuration options.  Currently the most widely used
free web server is Apache HTTPd, or Apache for short. Apache can be
easily installed on nearly every system using the system’s package
management tool.  lighttpd is another alternative and is said to have
better performance.  On many systems this server can also be installed
using the package management tool, so manually compiling the web
server may not be needed.

* On Apache you can take a look at the Dynamic Content with CGI
  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, which can be
  configured in a straightforward way.  It boils down to setting
  "cgi.assign" properly.


Common problems with CGI scripts
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Using CGI sometimes leads to small annoyances while trying to get
these scripts to run.  Sometimes a seemingly correct script does not
work as expected, the cause being some small hidden problem that’s
difficult to spot.

Some of these potential problems are:

* The Python script is not marked as executable.  When CGI scripts
  are not executable most 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 on Unix-like operating systems, the "+x" bit
  needs to be set. Using "chmod a+x your_script.py" may solve this
  problem.

* On a Unix-like system, The line endings in the program file must
  be Unix style line endings.  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 editors support this.

* Your web server must be able to read the file, and you need to
  make sure the permissions are correct.  On unix-like systems, the
  server often 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 web server must know that the file you’re trying to access is
  a CGI script. Check the configuration of your web server, as it may
  be configured to expect a specific file extension for CGI scripts.

* On Unix-like systems, the path to the interpreter in the shebang
  ("#!/usr/bin/env python") must be correct.  This line calls
  "/usr/bin/env" to find Python, but it will fail if there is no
  "/usr/bin/env", or if Python is not in the web server’s path.  If
  you know where your Python is installed, you can also use that full
  path.  The commands "whereis python" and "type -p python" could help
  you find where it is installed.  Once you know the path, you can
  change the shebang 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 and UTF-32 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.

* If the web server is using mod_python, "mod_python" may be having
  problems.  "mod_python" is able to handle CGI scripts by itself, but
  it can also be a source of issues.


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, because they
think that this is the equivalent to "mod_php".  Actually, there are
many differences.  What "mod_python" does is embed the interpreter
into the Apache process, thus speeding up requests by not having to
start a Python interpreter for each request.  On the other hand, it is
not “Python intermixed with HTML” in the way that PHP is often
intermixed with HTML. The Python equivalent of that is a template
engine.  "mod_python" itself is much more powerful and provides more
access to Apache internals.  It can emulate CGI, work in a “Python
Server Pages” mode (similar to JSP) which is “HTML intermingled with
Python”, and it has a “Publisher” which designates one file to accept
all requests and decide what to do with them.

"mod_python" does have some problems.  Unlike the PHP interpreter, the
Python interpreter uses caching when executing files, so changes to a
file will require the web server to be restarted.  Another problem is
the basic concept – Apache starts 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, because
"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 still might be 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 background processes. There is still a 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 of the server, it can be written in any language,
including Python.  The language just needs to have a library which
handles the communication with the webserver.

The difference between FastCGI and SCGI is very small, as SCGI is
essentially just a “simpler FastCGI”.  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 cover the latter.

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


Setting up FastCGI
~~~~~~~~~~~~~~~~~~

Each web server requires a specific module.

* Apache has both mod_fastcgi and mod_fcgid.  "mod_fastcgi" is the
  original one, but it has some licensing issues, which is 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 as well as an SCGI module.

* nginx also supports FastCGI.

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 first,
as flup handles the low level FastCGI access.

See also: There is some documentation on setting up Django with
  WSGI, 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 is an attempt to get rid of the low level gateways.  Given
that FastCGI, SCGI, and mod_python are mostly used to deploy WSGI
applications, mod_wsgi was started to directly embed WSGI applications
into the Apache web server. mod_wsgi is specifically designed to host
WSGI applications.  It makes the deployment of WSGI applications much
easier than deployment using other low level methods, which need glue
code.  The downside is that mod_wsgi is limited to the Apache web
server; other servers would need their own implementations of
mod_wsgi.

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


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

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

The *Web Server Gateway Interface*,  or WSGI for short, is defined in
**PEP 333** and is currently the best way to do Python web
programming.  While it is great for programmers writing frameworks, a
normal web developer does not need to get in direct contact with it.
When choosing a framework for web development it is a good idea to
choose one which supports WSGI.

The big benefit of WSGI is the unification of the application
programming interface.  When your program is compatible with WSGI –
which at the outer level means that the framework you are using has
support for WSGI – your program can be deployed via any web server
interface for which there are WSGI wrappers.  You do not need to care
about whether the application user uses mod_python or FastCGI or
mod_wsgi – with WSGI your application will work on any gateway
interface.  The Python standard library contains its own WSGI server,
"wsgiref", which is a small web server that can be used for testing.

A really great WSGI feature is middleware.  Middleware is a layer
around your program which can add various functionality to it.  There
is quite a bit of middleware already available.  For example, instead
of writing your own session management (HTTP is a stateless protocol,
so to associate multiple HTTP requests with a single user your
application must create and manage such state via a session), you can
just download middleware which does that, plug it in, and get on with
coding the unique parts of your application.  The same thing with
compression – there is existing middleware which handles compressing
your HTML using gzip to save on your server’s bandwidth.
Authentication is another problem that is easily solved using existing
middleware.

Although WSGI may seem complex, the initial phase of learning can be
very rewarding because WSGI and the associated middleware already have
solutions to many problems that might arise while developing web
sites.


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

The code that is used to connect to various low level gateways like
CGI or mod_python is called a *WSGI server*.  One of these servers is
"flup", which supports FastCGI and SCGI, as well as AJP.  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 many servers already available, so a Python web application
can be deployed nearly anywhere.  This is one big advantage that
Python has compared with other web technologies.

See also: A good overview of WSGI-related code can be found in the
  WSGI homepage, which contains an extensive list of WSGI servers
  which can be used by *any* application supporting WSGI.

  You might be interested in some WSGI-supporting modules already
  contained in the standard library, namely:

  * "wsgiref" – some tiny utilities and servers for WSGI


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

What does WSGI give the web application developer?  Let’s take a look
at an application that’s been around for a while, which was written in
Python without using WSGI.

One of the most widely used wiki software packages is MoinMoin.  It
was created in 2000, so it predates WSGI by about three years.  Older
versions needed separate code to run on CGI, mod_python, FastCGI and
standalone.

It now includes support for WSGI.  Using WSGI, it is possible to
deploy MoinMoin on any WSGI compliant server, with no additional glue
code. Unlike the pre-WSGI versions, this could include WSGI servers
that the authors of MoinMoin know nothing about.


Model-View-Controller
=====================

The term *MVC* is often encountered in statements such as “framework
*foo* supports MVC”.  MVC is more about the overall organization of
code, rather than any particular API.  Many web frameworks use this
model to help the developer bring structure to their program.  Bigger
web applications can have lots of code, so it is a good idea to have
an effective structure right from the beginning. That way, even users
of other frameworks (or even other languages, since MVC is not Python-
specific) can easily understand the code, given that they are already
familiar with the MVC structure.

MVC stands for three components:

* The *model*.  This is the data that will be displayed and
  modified. In Python frameworks, this component is often represented
  by the classes used by an object-relational mapper.

* The *view*.  This component’s job is to display the data of the
  model to the user.  Typically this component is implemented via
  templates.

* The *controller*.  This is the layer between the user and the
  model. The controller reacts to user actions (like opening some
  specific URL), tells the model to modify the data if necessary, and
  tells the view code what to display,

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
  separating 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 unnecessary Python code in the templates –
  it works against the MVC model and creates chaos in the code base,
  making it harder to understand and modify.

See also: The English Wikipedia has an article about the Model-View-
  Controller pattern.  It includes a long list of web frameworks for
  various programming languages.


Ingredients for Websites
========================

Websites are complex constructs, so tools have been created to help
web developers make their code easier to write and more maintainable.
Tools like these exist for all web frameworks in all languages.
Developers are not forced to use these tools, and often there is no
“best” tool.  It is worth learning about the available tools because
they can greatly simplify the process of developing a web site.

See also: There are far more components than can be presented here.
  The Python wiki has a page about these components, called Web
  Components.


Templates
---------

Mixing of HTML and Python code is made possible by a few 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 in the placeholders.

Python already includes two ways to build simple templates:

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

   >>> from string import Template
   >>> template = Template("<html><body><h1>Hello ${name}</h1></body></html>")
   >>> print template.substitute(dict(name='Dinsdale'))
   <html><body><h1>Hello Dinsdale!</h1></body></html>

To generate complex HTML based on non-trivial model data, conditional
and looping constructs like Python’s *for* and *if* are generally
needed. *Template engines* support templates of this complexity.

There are a lot of template engines available for Python which can be
used with or without a framework.  Some of these define a plain-text
programming language which is easy to learn, partly because it is
limited in scope. Others use XML, and the template output is
guaranteed to be always be valid XML.  There are many other
variations.

Some frameworks ship their own template engine or recommend one in
particular.  In the absence of a reason to use a different template
engine, using the one provided by or recommended by the framework is a
good idea.

Popular template engines include:

   * Mako

   * Genshi

   * Jinja

See also: There are many template engines competing for attention,
  because it is pretty easy to create them in Python.  The page
  Templating in the wiki lists a big, ever-growing number of these.
  The three listed above are considered “second generation” template
  engines and are a good place to start.


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

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

Often, relational database engines like MySQL or PostgreSQL are used
because of their good performance when handling very large databases
consisting of millions of entries.  There is also a small database
engine called SQLite, which is bundled with Python in the "sqlite3"
module, and which uses only one file.  It has no other dependencies.
For smaller sites SQLite is just enough.

Relational databases are *queried* using a language called SQL.
Python programmers in general do not like SQL too much, as they prefer
to work with objects.  It is possible to save Python objects into a
database using a technology called ORM (Object Relational Mapping).
ORM translates all object-oriented access into SQL code under the
hood, so the developer does not need to think about it.  Most
frameworks use ORMs, and it works quite well.

A second possibility is storing data in normal, plain text files (some
times called “flat files”).  This is very easy for simple sites, but
can be difficult to get right if the web site is performing many
updates to the stored data.

A third possibility are object oriented databases (also called “object
databases”).  These databases store the object data in a form that
closely parallels the way the objects are structured in memory during
program execution.  (By contrast, ORMs store the object data as rows
of data in tables and relations between those rows.)  Storing the
objects directly has the advantage that nearly all objects can be
saved in a straightforward way, unlike in relational databases where
some objects are very hard to represent.

Frameworks often give hints on which data storage method to choose.
It is usually a good idea to stick to the data store recommended by
the framework unless the application has special requirements better
satisfied by an alternate storage mechanism.

See also:

  * Persistence Tools lists possibilities on how to save data in the
    file system.  Some of these modules are part of the standard
    library

  * Database Programming helps with choosing a method for saving
    data

  * SQLAlchemy, the most powerful OR-Mapper for Python, and Elixir,
    which makes SQLAlchemy easier to use

  * SQLObject, another popular OR-Mapper

  * ZODB and Durus, two object oriented databases


Frameworks
==========

The process of creating code to run web sites involves writing code to
provide various services.  The code to provide a particular service
often works the same way regardless of the complexity or purpose of
the web site in question. Abstracting these common solutions into
reusable code produces what are called “frameworks” for web
development.  Perhaps the most well-known framework for web
development is Ruby on Rails, but Python has its own frameworks.  Some
of these were partly inspired by Rails, or borrowed ideas from Rails,
but many existed a long time before Rails.

Originally Python web frameworks tended to incorporate all of the
services needed to develop web sites as a giant, integrated set of
tools.  No two web frameworks were interoperable:  a program developed
for one could not be deployed on a different one without considerable
re-engineering work.  This led to the development of “minimalist” web
frameworks that provided just the tools to communicate between the
Python code and the http protocol, with all other services to be added
on top via separate components.  Some ad hoc standards were developed
that allowed for limited interoperability between frameworks, such as
a standard that allowed different template engines to be used
interchangeably.

Since the advent of WSGI, the Python web framework world has been
evolving toward interoperability based on the WSGI standard.  Now many
web frameworks, whether “full stack” (providing all the tools one
needs to deploy the most complex web sites) or minimalist, or anything
in between, are built from collections of reusable components that can
be used with more than one framework.

The majority of users will probably want to select a “full stack”
framework that has an active community.  These frameworks tend to be
well documented, and provide the easiest path to producing a fully
functional web site in minimal time.


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

There are an incredible number of frameworks, so they cannot all be
covered here.  Instead we will briefly touch on some of the most
popular.


Django
~~~~~~

Django 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 template inheritance and filters (which
work like Unix pipes).  Django has many handy features bundled, such
as creation of RSS feeds or generic views, which make it possible to
create web sites almost without writing any Python code.

It has a big, international community, the members of which have
created many web sites.  There are also a lot of add-on projects which
extend Django’s normal functionality.  This is partly due to Django’s
well written online documentation and the Django book.

Note: Although Django is an MVC-style framework, it names the
  elements differently, which is described in the Django FAQ.


TurboGears
~~~~~~~~~~

Another popular web framework for Python is TurboGears.  TurboGears
takes the approach of using already existing components and combining
them with glue code to create a seamless experience.  TurboGears gives
the user flexibility in choosing components. For example the ORM and
template engine can be changed to use packages different from those
used by default.

The documentation can be found in the TurboGears documentation, 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 published, which is a good starting point.

The newest version of TurboGears, version 2.0, moves even further in
direction of WSGI support and a component-based architecture.
TurboGears 2 is based on the WSGI stack of another popular component-
based web framework, Pylons.


Zope
~~~~

The Zope framework is one of the “old original” frameworks.  Its
current incarnation in Zope2 is a tightly integrated full-stack
framework.  One of its most interesting feature is its tight
integration with a powerful object database called the ZODB (Zope
Object Database). Because of its highly integrated nature, Zope wound
up in a somewhat isolated ecosystem:  code written for Zope wasn’t
very usable outside of Zope, and vice-versa.  To solve this problem
the Zope 3 effort was started.  Zope 3 re-engineers Zope as a set of
more cleanly isolated components.  This effort was started before the
advent of the WSGI standard, but there is WSGI support for Zope 3 from
the Repoze project.  Zope components have many years of production use
behind them, and the Zope 3 project gives access to these components
to the wider Python community.  There is even a separate framework
based on the Zope components: Grok.

Zope is also the infrastructure used by the Plone content management
system, one of the most powerful and popular content management
systems available.


Other notable frameworks
~~~~~~~~~~~~~~~~~~~~~~~~

Of course these are not the only frameworks that are available.  There
are many other frameworks worth mentioning.

Another framework that’s already been mentioned is Pylons.  Pylons is
much like TurboGears, but with an even stronger emphasis on
flexibility, which comes at the cost of being more difficult to use.
Nearly every component can be exchanged, which makes it necessary to
use the documentation of every single component, of which there are
many.  Pylons builds upon Paste, 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.

See also: The Python wiki contains an extensive list of web
  frameworks.

  Most frameworks also have their own mailing lists and IRC channels,
  look out for these on the projects’ web sites.
