modpython.txt

=================================
How to use Django with mod_python
=================================

Apache_ with `mod_python`_ currently is the preferred setup for using Django
on a production server.

mod_python is similar to `mod_perl`_ : It embeds Python within Apache and loads
Python code into memory when the server starts. Code stays in memory throughout
the life of an Apache process, which leads to significant performance gains over
other server arrangements.

Django requires Apache 2.x and mod_python 3.x, and you should use Apache's
`prefork MPM`_, as opposed to the `worker MPM`_.

You may also be interested in `How to use Django with FastCGI, SCGI or AJP`_
(which also covers SCGI and AJP).

.. _Apache: http://httpd.apache.org/
.. _mod_python: http://www.modpython.org/
.. _mod_perl: http://perl.apache.org/
.. _prefork MPM: http://httpd.apache.org/docs/2.2/mod/prefork.html
.. _worker MPM: http://httpd.apache.org/docs/2.2/mod/worker.html
.. _How to use Django with FastCGI, SCGI or AJP: ../fastcgi/

Basic configuration
===================

To configure Django with mod_python, first make sure you have Apache installed,
with the mod_python module activated.

Then edit your ``httpd.conf`` file and add the following::

    <Location "/mysite/">
        SetHandler python-program
        PythonHandler django.core.handlers.modpython
        SetEnv DJANGO_SETTINGS_MODULE mysite.settings
        PythonDebug On
    </Location>

...and replace ``mysite.settings`` with the Python import path to your Django
project's settings file.

This tells Apache: "Use mod_python for any URL at or under '/mysite/', using the
Django mod_python handler." It passes the value of ``DJANGO_SETTINGS_MODULE``
so mod_python knows which settings to use.

Note that we're using the ``<Location>`` directive, not the ``<Directory>``
directive. The latter is used for pointing at places on your filesystem,
whereas ``<Location>`` points at places in the URL structure of a Web site.
``<Directory>`` would be meaningless here.

Also, if your Django project is not on the default ``PYTHONPATH`` for your
computer, you'll have to tell mod_python where your project can be found:

.. parsed-literal::

    <Location "/mysite/">
        SetHandler python-program
        PythonHandler django.core.handlers.modpython
        SetEnv DJANGO_SETTINGS_MODULE mysite.settings
        PythonDebug On
        **PythonPath "['/path/to/project'] + sys.path"**
    </Location>

The value you use for ``PythonPath`` should include the parent directories of
all the modules you are going to import in your application. It should also
include the parent directory of the ``DJANGO_SETTINGS_MODULE`` location. This
is exactly the same situation as setting the Python path for interactive
usage. Whenever you try to import something, Python will run through all the
directories in ``sys.path`` in turn, from first to last, and try to import
from each directory until one succeeds.

An example might make this clearer. Suppose
you have some applications under ``/usr/local/django-apps/`` (for example,
``/usr/local/django-apps/weblog/`` and so forth), your settings file is at
``/var/www/mysite/settings.py`` and you have specified
``DJANGO_SETTINGS_MODULE`` as in the above example. In this case, you would
need to write your ``PythonPath`` directive as::

    PythonPath "['/var/production/django-apps/', '/var/www'] + sys.path"

With this path, ``import weblog`` and ``import mysite.settings`` will both
work. If you had ``import blogroll`` in your code somewhere and ``blogroll``
lived under the ``weblog/`` directory, you would *also* need to add
``/var/production/django-apps/weblog/`` to your ``PythonPath``. Remember: the
**parent directories** of anything you import directly must be on the Python
path.

.. caution::

    If you're using Windows, remember that the path will contain backslashes.
    This string is passed through Python's string parser twice, so you need to
    escape each backslash **twice**::

        PythonPath "['c:\\\\path\\\\to\\\\project'] + sys.path"

    Or, use raw strings::

        PythonPath "[r'c:\\path\\to\\project'] + sys.path"

You can also add directives such as ``PythonAutoReload Off`` for performance.
See the `mod_python documentation`_ for a full list of options.

Note that you should set ``PythonDebug Off`` on a production server. If you
leave ``PythonDebug On``, your users would see ugly (and revealing) Python
tracebacks if something goes wrong within mod_python.

Restart Apache, and any request to /mysite/ or below will be served by Django.
Note that Django's URLconfs won't trim the "/mysite/" -- they get passed the
full URL.

When deploying Django sites on mod_python, you'll need to restart Apache each
time you make changes to your Python code.

Multiple Django installations on the same Apache
================================================

It's entirely possible to run multiple Django installations on the same Apache
instance. Just use ``VirtualHost`` for that, like so::

    NameVirtualHost *

    <VirtualHost *>
        ServerName www.example.com
        # ...
        SetEnv DJANGO_SETTINGS_MODULE mysite.settings
    </VirtualHost>

    <VirtualHost *>
        ServerName www2.example.com
        # ...
        SetEnv DJANGO_SETTINGS_MODULE mysite.other_settings
    </VirtualHost>

If you need to put two Django installations within the same ``VirtualHost``,
you'll need to take a special precaution to ensure mod_python's cache doesn't
mess things up. Use the ``PythonInterpreter`` directive to give different
``<Location>`` directives separate interpreters::

    <VirtualHost *>
        ServerName www.example.com
        # ...
        <Location "/something">
            SetEnv DJANGO_SETTINGS_MODULE mysite.settings
            PythonInterpreter mysite
        </Location>

        <Location "/otherthing">
            SetEnv DJANGO_SETTINGS_MODULE mysite.other_settings
            PythonInterpreter mysite_other
        </Location>
    </VirtualHost>

The values of ``PythonInterpreter`` don't really matter, as long as they're
different between the two ``Location`` blocks.

Running a development server with mod_python
============================================

If you use mod_python for your development server, you can avoid the hassle of
having to restart the server each time you make code changes. Just set
``MaxRequestsPerChild 1`` in your ``httpd.conf`` file to force Apache to reload
everything for each request. But don't do that on a production server, or we'll
revoke your Django privileges.

If you're the type of programmer who debugs using scattered ``print``
statements, note that ``print`` statements have no effect in mod_python; they
don't appear in the Apache log, as one might expect. If you have the need to
print debugging information in a mod_python setup, either do this::

    assert False, the_value_i_want_to_see

Or add the debugging information to the template of your page.

.. _mod_python documentation: http://modpython.org/live/current/doc-html/directives.html

Serving media files
===================

Django doesn't serve media files itself; it leaves that job to whichever Web
server you choose.

We recommend using a separate Web server -- i.e., one that's not also running
Django -- for serving media. Here are some good choices:

* lighttpd_
* TUX_
* A stripped-down version of Apache_

If, however, you have no option but to serve media files on the same Apache
``VirtualHost`` as Django, here's how you can turn off mod_python for a
particular part of the site::

    <Location "/media">
        SetHandler None
    </Location>

Just change ``Location`` to the root URL of your media files. You can also use
``<LocationMatch>`` to match a regular expression.

This example sets up Django at the site root but explicitly disables Django for
the ``media`` subdirectory and any URL that ends with ``.jpg``, ``.gif`` or
``.png``::

    <Location "/">
        SetHandler python-program
        PythonHandler django.core.handlers.modpython
        SetEnv DJANGO_SETTINGS_MODULE mysite.settings
    </Location>

    <Location "/media">
        SetHandler None
    </Location>

    <LocationMatch "\.(jpg|gif|png)$">
        SetHandler None
    </LocationMatch>


.. _lighttpd: http://www.lighttpd.net/
.. _TUX: http://en.wikipedia.org/wiki/TUX_web_server
.. _Apache: http://httpd.apache.org/

Serving the admin files
=======================

Note that the Django development server automagically serves admin media files,
but this is not the case when you use any other server arrangement. You're
responsible for setting up Apache, or whichever media server you're using, to
serve the admin files.

The admin files live in (``django/contrib/admin/media``) of the Django
distribution.

Here are two recommended approaches:

    1. Create a symbolic link to the admin media files from within your
       document root. This way, all of your Django-related files -- code
       **and** templates -- stay in one place, and you'll still be able to
       ``svn update`` your code to get the latest admin templates, if they
       change.
    2. Or, copy the admin media files so that they live within your Apache
       document root.

Using eggs with mod_python
==========================

If you installed Django from a Python egg_ or are using eggs in your Django
project, some extra configuration is required. Create an extra file in your
project (or somewhere else) that contains something like the following::

    import os
    os.environ['PYTHON_EGG_CACHE'] = '/some/directory'

Here, ``/some/directory`` is a directory that the Apache webserver process can
write to. It will be used as the location for any unpacking of code the eggs
need to do.

Then you have to tell mod_python to import this file before doing anything
else. This is done using the PythonImport_ directive to mod_python. You need
to ensure that you have specified the ``PythonInterpreter`` directive to
mod_python as described above__ (you need to do this even if you aren't
serving multiple installations in this case). Then add the ``PythonImport``
line in the main server configuration (i.e., outside the ``Location`` or
``VirtualHost`` sections). For example::

    PythonInterpreter my_django
    PythonImport /path/to/my/project/file.py my_django

Note that you can use an absolute path here (or a normal dotted import path),
as described in the `mod_python manual`_. We use an absolute path in the
above example because if any Python path modifications are required to access
your project, they will not have been done at the time the ``PythonImport``
line is processed.

.. _Egg: http://peak.telecommunity.com/DevCenter/PythonEggs
.. _PythonImport: http://www.modpython.org/live/current/doc-html/dir-other-pimp.html
.. _mod_python manual: PythonImport_
__ `Multiple Django installations on the same Apache`_

Error handling
==============

When you use Apache/mod_python, errors will be caught by Django -- in other
words, they won't propagate to the Apache level and won't appear in the Apache
``error_log``.

The exception for this is if something is really wonky in your Django setup. In
that case, you'll see an "Internal Server Error" page in your browser and the
full Python traceback in your Apache ``error_log`` file. The ``error_log``
traceback is spread over multiple lines. (Yes, this is ugly and rather hard to
read, but it's how mod_python does things.)

If you get a segmentation fault
===============================

If Apache causes a segmentation fault, there are two probable causes, neither
of which has to do with Django itself.

    1. It may be because your Python code is importing the "pyexpat" module,
       which may conflict with the version embedded in Apache. For full
       information, see `Expat Causing Apache Crash`_.
    2. It may be because you're running mod_python and mod_php in the same
       Apache instance, with MySQL as your database backend. In some cases,
       this causes a known mod_python issue due to version conflicts in PHP and
       the Python MySQL backend. There's full information in the
       `mod_python FAQ entry`_.

If you continue to have problems setting up mod_python, a good thing to do is
get a barebones mod_python site working, without the Django framework. This is
an easy way to isolate mod_python-specific problems. `Getting mod_python Working`_
details this procedure.

The next step should be to edit your test code and add an import of any
Django-specific code you're using -- your views, your models, your URLconf,
your RSS configuration, etc. Put these imports in your test handler function
and access your test URL in a browser. If this causes a crash, you've confirmed
it's the importing of Django code that causes the problem. Gradually reduce the
set of imports until it stops crashing, so as to find the specific module that
causes the problem. Drop down further into modules and look into their imports,
as necessary.

.. _Expat Causing Apache Crash: http://www.dscpl.com.au/articles/modpython-006.html
.. _mod_python FAQ entry: http://modpython.org/FAQ/faqw.py?req=show&file=faq02.013.htp
.. _Getting mod_python Working: http://www.dscpl.com.au/articles/modpython-001.html