[Python-checkins] r79784 - in python/branches/release31-maint: Doc/howto/webservers.rst
ezio.melotti
python-checkins at python.org
Mon Apr 5 06:06:12 CEST 2010
Author: ezio.melotti
Date: Mon Apr 5 06:06:12 2010
New Revision: 79784
Log:
Merged revisions 79783 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k
................
r79783 | ezio.melotti | 2010-04-05 07:04:35 +0300 (Mon, 05 Apr 2010) | 9 lines
Merged revisions 79781 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
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r79781 | ezio.melotti | 2010-04-05 06:51:38 +0300 (Mon, 05 Apr 2010) | 1 line
#8212: rephrase the webservers howto and fix some mistakes.
........
................
Modified:
python/branches/release31-maint/ (props changed)
python/branches/release31-maint/Doc/howto/webservers.rst
Modified: python/branches/release31-maint/Doc/howto/webservers.rst
==============================================================================
--- python/branches/release31-maint/Doc/howto/webservers.rst (original)
+++ python/branches/release31-maint/Doc/howto/webservers.rst Mon Apr 5 06:06:12 2010
@@ -6,74 +6,75 @@
.. 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
+ 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 raise of the "Web 2.0",
+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 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.
+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.
.. 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
+ 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
+ <http://wiki.python.org/moin/WebProgramming>`_ may be more in sync with
recent development.
-The low-level view
+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
+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.
-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.
+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 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.
+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
+:ref:`WSGI <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.
.. seealso::
@@ -81,7 +82,7 @@
creating plain CGI programs:
* :mod:`cgi` -- Handling of user input in CGI scripts
- * :mod:`cgitb` -- Displays nice tracebacks when errors happen in of CGI
+ * :mod:`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
@@ -107,16 +108,15 @@
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.
+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 confident data to the user. Don't use it when the script is
-ready for production use. Still, you should *always* catch exceptions, and
+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.
@@ -125,73 +125,83 @@
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.
+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 <http://httpd.apache.org/>`_, or Apache for short. Apache can be
+easily installed on nearly every system using the system's package management
+tool. `lighttpd <http://www.lighttpd.net>`_ 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 into the `Dynamic Content with CGI
+* On Apache you can take a look at 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
+ <http://redmine.lighttpd.net/wiki/lighttpd/Docs:ModCGI>`_\ , 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.
+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 reasons are:
+Some of these potential problems 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
+* 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 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``.
+ 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 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.
+ 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 :ref:`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:
@@ -200,33 +210,34 @@
----------
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.
+Their first thought is mostly `mod_python <http://www.modpython.org/>`_\ ,
+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
@@ -234,19 +245,19 @@
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.
+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". But as the web server support for SCGI is limited
+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 write about
+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
+These days, FastCGI is never used directly. Just like ``mod_python``, it is only
used for the deployment of WSGI applications.
.. seealso::
@@ -260,17 +271,20 @@
Setting up FastCGI
^^^^^^^^^^^^^^^^^^
-Depending on the web server you need to have a special module.
+Each web server requires a specific module.
-* Apache has both `mod_fastcgi <http://www.fastcgi.com/>`_ and `mod_fcgid
+* Apache has both `mod_fastcgi <http://www.fastcgi.com/drupal/>`_ 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
+ 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
- <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.nginx.org/NginxSimplePythonFCGI>`_.
+ <http://redmine.lighttpd.net/wiki/lighttpd/Docs:ModFastCGI>`_ as well as an
+ `SCGI module <http://redmine.lighttpd.net/wiki/lighttpd/Docs:ModSCGI>`_.
+
+* `nginx <http://nginx.org/>`_ also supports `FastCGI
+ <http://wiki.nginx.org/NginxSimplePythonFCGI>`_.
Once you have installed and configured the module, you can test it with the
following WSGI-application::
@@ -301,28 +315,27 @@
.. 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.
+ <http://docs.djangoproject.com/en/dev/howto/deployment/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
+`mod_wsgi <http://code.google.com/p/modwsgi/>`_ 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.
-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
+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.
@@ -331,58 +344,61 @@
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.
+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*, :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.
+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, :mod:`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 <http://wsgi.org/wsgi/Middleware_and_Utilities>`_ 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 a problem 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 *WSGI server*. One of these servers is ``flup`` which was
-already mentioned and supports FastCGI, SCGI as well as `AJP
+mod_python is called a *WSGI server*. One of these servers is ``flup``, which
+supports FastCGI and 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
+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.
+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.
.. seealso::
- A good overview of all WSGI-related code can be found in the `WSGI wiki
+ A good overview of 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
+ <http://wsgi.org/wsgi/Servers>`_ which can be used by *any* application
supporting WSGI.
You might be interested in some WSGI-supporting modules already contained in
@@ -394,39 +410,46 @@
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.
+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
+<http://moinmo.in/>`_. 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
+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.
+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 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 *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 represented by the templates.
+ user. Typically this component is implemented via 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 necessary.
+ 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,
@@ -438,132 +461,136 @@
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 is against MVC and creates more chaos.
+ 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.
.. 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.
+ The English Wikipedia has an article about the `Model-View-Controller pattern
+ <http://en.wikipedia.org/wiki/Model-view-controller>`_. It includes a long
+ list of web frameworks for various 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.
+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.
.. 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
+ There are far more components than can be 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
+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 out the
+placeholders. The HTML is sent to the user's browser after filling in the
placeholders.
-Python already includes such simple templates::
+Python already includes a way to build simple templates::
# a simple template
template = "<html><body><h1>Hello {who}!</h1></body></html>"
print(template.format(who="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
+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 <http://www.makotemplates.org/>`_
+ * `Genshi <http://genshi.edgewall.org/>`_
+ * `Jinja <http://jinja.pocoo.org/2/>`_
.. seealso::
- Lots of different template engines divide the attention between themselves
- because it's easy to create them in Python. The page `Templating
+ There are many template engines competing for attention, becuase it is
+ pretty 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.
+ 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 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://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.
+*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 <http://www.mysql.com/>`_ or
+`PostgreSQL <http://www.postgresql.org/>`_ 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
+<http://www.sqlite.org/>`_, which is bundled with Python in the :mod:`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
+<http://en.wikipedia.org/wiki/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
+<http://en.wikipedia.org/wiki/Object-relational_mapping>`_ (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.
.. 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
+ 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
+ helps with choosing a method for saving data
+
+ * `SQLAlchemy <http://www.sqlalchemy.org/>`_, the most powerful OR-Mapper
+ for Python, and `Elixir <http://elixir.ematia.de/>`_, which makes
+ SQLAlchemy 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
@@ -574,42 +601,44 @@
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.
+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 is an incredible number of frameworks, so there is no way to describe them
-all. It is not even necessary, 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.
+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
@@ -617,16 +646,16 @@
`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.
+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 which has created many sites using Django.
-There are also quite a lot of add-on projects which extend Django's normal
+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 <http://docs.djangoproject.com/>`_ and the `Django book
<http://www.djangobook.com/>`_.
@@ -634,22 +663,20 @@
.. note::
- Although Django is an MVC-style framework, it calls the components
+ Although Django is an MVC-style framework, it names the elements
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>`_.
+ <http://docs.djangoproject.com/en/dev/faq/general/#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.
+Another popular web framework for Python is `TurboGears
+<http://www.turbogears.org/>`_. 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 wiki
<http://docs.turbogears.org/>`_, where links to screencasts can be found.
@@ -657,31 +684,46 @@
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/>`_.
+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
+<http://pylonshq.com/>`_.
+
+
+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 <https://launchpad.net/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 <http://repoze.org/>`_ 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
+<http://grok.zope.org/>`_.
+
+Zope is also the infrastructure used by the `Plone <http://plone.org/>`_ content
+management system, one of the most powerful and popular content management
+systems available.
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.
+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 with an even stronger emphasis on flexibility, which is bought
+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, because there are so many Pylons combinations possible that can
-satisfy every requirement. Pylons builds upon `Paste
+component, of which there are many. 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
@@ -693,6 +735,6 @@
<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
+ for these on the projects' web sites. There is also a general "Python in the
Web" IRC channel on freenode called `#python.web
<http://wiki.python.org/moin/PoundPythonWeb>`_.
More information about the Python-checkins
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