root/django/branches/0.90-bugfixes/docs/syndication_feeds.txt

==============================
The syndication feed framework
==============================

Django comes with a high-level syndication-feed-generating framework that makes
creating RSS_ and Atom_ feeds easy.

To create any syndication feed, all you have to do is write a short Python
class. You can create as many feeds as you want.

Django also comes with a lower-level feed-generating API. Use this if you want
to generate feeds outside of a Web context, or in some other lower-level way.

.. _RSS: http://www.whatisrss.com/
.. _Atom: http://www.atomenabled.org/

The high-level framework
========================

Overview
--------

The high-level feed-generating framework is a view that's hooked to ``/feeds/``
by default. Django uses the remainder of the URL (everything after ``/feeds/``)
to determine which feed to output.

To create a feed, just write a ``Feed`` class and point to it in your URLconf_.

.. _URLconf: http://www.djangoproject.com/documentation/url_dispatch/

Initialization
--------------

To activate syndication feeds on your Django site, add this line to your
URLconf_::

    (r'^feeds/(?P<url>.*)/$', 'django.contrib.syndication.views.feed', {'feed_dict': feeds}),

This tells Django to use the RSS framework to handle all URLs starting with
``"feeds/"``. (You can change that ``"feeds/"`` prefix to fit your own needs.)

This URLconf line has an extra argument: ``{'feed_dict': feeds}``. Use this
extra argument to pass the syndication framework the feeds that should be
published under that URL.

Specifically, ``feed_dict`` should be a dictionary that maps a feed's slug
(short URL label) to its ``Feed`` class.

You can define the ``feed_dict`` in the URLconf itself. Here's a full example
URLconf::

    from django.conf.urls.defaults import *
    from myproject.feeds import LatestEntries, LatestEntriesByCategory

    feeds = {
        'latest': LatestEntries,
        'categories': LatestEntriesByCategory,
    }

    urlpatterns = patterns('',
        # ...
        (r'^feeds/(?P<url>.*)/$', 'django.contrib.syndication.views.feed',
            {'feed_dict': feeds}),
        # ...
    )

The above example registers two feeds:

    * The feed represented by ``LatestEntries`` will live at ``feeds/latest/``.
    * The feed represented by ``LatestEntriesByCategory`` will live at
      ``feeds/categories/``.

Once that's set up, you just need to define the ``Feed`` classes themselves.

.. _URLconf: http://www.djangoproject.com/documentation/url_dispatch/
.. _settings file: http://www.djangoproject.com/documentation/settings/

Feed classes
------------

A ``Feed`` class is a simple Python class that represents a syndication feed.
A feed can be simple (e.g., a "site news" feed, or a basic feed displaying
the latest entries of a blog) or more complex (e.g., a feed displaying all the
blog entries in a particular category, where the category is variable).

``Feed`` classes must subclass ``django.contrib.syndication.feeds.Feed``. They
can live anywhere in your codebase.

A simple example
----------------

This simple example, taken from `chicagocrime.org`_, describes a feed of the
latest five news items::

    from django.contrib.syndication.feeds import Feed
    from django.models.chicagocrime import newsitems

    class SiteNewsFeed(Feed):
        title = "Chicagocrime.org site news"
        link = "/sitenews/"
        description = "Updates on changes and additions to chicagocrime.org."

        def items(self):
            return newsitems.get_list(order_by=('-pub_date',), limit=5)

Note:

    * The class subclasses ``django.contrib.syndication.feeds.Feed``.
    * ``title``, ``link`` and ``description`` correspond to the standard
      RSS ``<title>``, ``<link>`` and ``<description>`` elements, respectively.
    * ``items()`` is, simply, a method that returns a list of objects that
      should be included in the feed as ``<item>`` elements. Although this
      example returns ``NewsItem`` objects using Django's
      `object-relational mapper`_, ``items()`` doesn't have to return model
      instances. Although you get a few bits of functionality "for free" by
      using Django models, ``items()`` can return any type of object you want.

One thing's left to do. In an RSS feed, each ``<item>`` has a ``<title>``,
``<link>`` and ``<description>``. We need to tell the framework what data to
put into those elements.

    * To specify the contents of ``<title>`` and ``<description>``, create
      `Django templates`_ called ``feeds/sitenews_title`` and
      ``feeds/sitenews_description``, where ``sitenews`` is the ``slug``
      specified in the URLconf for the given feed. The RSS system renders that
      template for each item, passing it two template context variables:

          * ``{{ obj }}`` -- The current object (one of whichever objects you
            returned in ``items()``).
          * ``{{ site }}`` -- A ``django.models.core.sites.Site`` object
            representing the current site. This is useful for
            ``{{ site.domain }}`` or ``{{ site.name }}``.

      If you don't create a template for either the title or description, the
      framework will use the template ``"{{ obj }}"`` by default -- that is,
      the normal string representation of the object.
    * To specify the contents of ``<link>``, you have two options. For each
      item in ``items()``, Django first tries executing a
      ``get_absolute_url()`` method on that object. If that method doesn't
      exist, it tries calling a method ``item_link()`` in the ``Feed`` class,
      passing it a single parameter, ``item``, which is the object itself.
      Both ``get_absolute_url()`` and ``item_link()`` should return the item's
      URL as a normal Python string.

.. _chicagocrime.org: http://www.chicagocrime.org/
.. _object-relational mapper: http://www.djangoproject.com/documentation/db_api/
.. _Django templates: http://www.djangoproject.com/documentation/templates/

A complex example
-----------------

The framework also supports more complex feeds, via parameters.

For example, `chicagocrime.org`_ offers an RSS feed of recent crimes for every
police beat in Chicago. It'd be silly to create a separate ``Feed`` class for
each police beat; that would violate the `DRY principle`_ and would couple data
to programming logic. Instead, the syndication framework lets you make generic
feeds that output items based on information in the feed's URL.

On chicagocrime.org, the police-beat feeds are accessible via URLs like this:

    * ``/rss/beats/0613/`` -- Returns recent crimes for beat 0613.
    * ``/rss/beats/1424/`` -- Returns recent crimes for beat 1424.

The slug here is ``"beats"``. The syndication framework sees the extra URL bits
after the slug -- ``0613`` and ``1424`` -- and gives you a hook to tell it what
those URL bits mean, and how they should influence which items get published in
the feed.

An example makes this clear. Here's the code for these beat-specific feeds::

    class BeatFeed(Feed):
        def get_object(self, bits):
            # In case of "/rss/beats/0613/foo/bar/baz/", or other such clutter,
            # check that bits has only one member.
            if len(bits) != 1:
                raise ObjectDoesNotExist
            return beats.get_object(beat__exact=bits[0])

        def title(self, obj):
            return "Chicagocrime.org: Crimes for beat %s" % obj.beat

        def link(self, obj):
            return obj.get_absolute_url()

        def description(self, obj):
            return "Crimes recently reported in police beat %s" % obj.beat

        def items(self, obj):
            return crimes.get_list(beat__id__exact=obj.id, order_by=(('-crime_date'),), limit=30)

Here's the basic algorithm the RSS framework follows, given this class and a
request to the URL ``/rss/beats/0613/``:

    * The framework gets the URL ``/rss/beats/0613/`` and notices there's
      an extra bit of URL after the slug. It splits that remaining string by
      the slash character (``"/"``) and calls the ``Feed`` class'
      ``get_object()`` method, passing it the bits. In this case, bits is
      ``['0613']``. For a request to ``/rss/beats/0613/foo/bar/``, bits would
      be ``['0613', 'foo', 'bar']``.
    * ``get_object()`` is responsible for retrieving the given beat, from the
      given ``bits``. In this case, it uses the Django database API to retrieve
      the beat. Note that ``get_object()`` should raise
      ``django.core.exceptions.ObjectDoesNotExist`` if given invalid
      parameters. There's no ``try``/``except`` around the
      ``beats.get_object()`` call, because it's not necessary; that function
      raises ``BeatDoesNotExist`` on failure, and ``BeatDoesNotExist`` is a
      subclass of ``ObjectDoesNotExist``. Raising ``ObjectDoesNotExist`` in
      ``get_object()`` tells Django to produce a 404 error for that request.
    * To generate the feed's ``<title>``, ``<link>`` and ``<description>``,
      Django uses the ``title()``, ``link()`` and ``description()`` methods. In
      the previous example, they were simple string class attributes, but this
      example illustrates that they can be either strings *or* methods. For
      each of ``title``, ``link`` and ``description``, Django follows this
      algorithm:

          * First, it tries to call a method, passing the ``obj`` argument, where
            ``obj`` is the object returned by ``get_object()``.
          * Failing that, it tries to call a method with no arguments.
          * Failing that, it uses the class attribute.

    * Finally, note that ``items()`` in this example also takes the ``obj``
      argument. The algorithm for ``items`` is the same as described in the
      previous step -- first, it tries ``items(obj)``, then ``items()``, then
      finally an ``items`` class attribute (which should be a list).

The ``ExampleFeed`` class below gives full documentation on methods and
attributes of ``Feed`` classes.

.. _DRY principle: http://c2.com/cgi/wiki?DontRepeatYourself

Specifying the type of feed
---------------------------

By default, feeds produced in this framework use RSS 2.0.

To change that, add a ``feed_type`` attribute to your ``Feed`` class, like so::

    from django.utils.feedgenerator import Atom1Feed

    class MyFeed(Feed):
        feed_type = Atom1Feed

Note that you set ``feed_type`` to a class object, not an instance.

Currently available feed types are:

    * ``django.utils.feedgenerator.Rss201rev2Feed`` (RSS 2.01. Default.)
    * ``django.utils.feedgenerator.RssUserland091Feed`` (RSS 0.91.)
    * ``django.utils.feedgenerator.Atom1Feed`` (Atom 1.0.)

Enclosures
----------

To specify enclosures, such as those used in creating podcast feeds, use the
``item_enclosure_url``, ``item_enclosure_length`` and
``item_enclosure_mime_type`` hooks. See the ``ExampleFeed`` class below for
usage examples.

Language
--------

Feeds created by the syndication framework automatically include the
appropriate ``<language>`` tag (RSS 2.0) or ``xml:lang`` attribute (Atom). This
comes directly from your `LANGUAGE_CODE setting`_.

.. _LANGUAGE_CODE setting: http://www.djangoproject.com/documentation/settings/#language-code

URLs
----

The ``link`` method/attribute can return either an absolute URL (e.g.
``"/blog/"``) or a URL with the fully-qualified domain and protocol (e.g.
``"http://www.example.com/blog/"``). If ``link`` doesn't return the domain,
the syndication framework will insert the domain of the current site, according
to your `SITE_ID setting`_.

Atom feeds require a ``<link rel="self">`` that defines the feed's current
location. The syndication framework populates this automatically, using the
domain of the current site according to the SITE_ID setting.

.. _SITE_ID setting: http://www.djangoproject.com/documentation/settings/#site-id

Publishing Atom and RSS feeds in tandem
---------------------------------------

Some developers like to make available both Atom *and* RSS versions of their
feeds. That's easy to do with Django: Just create a subclass of your ``feed``
class and set the ``feed_type`` to something different. Then update your
URLconf to add the extra versions.

Here's a full example::

    from django.contrib.syndication.feeds import Feed
    from django.models.chicagocrime import newsitems
    from django.utils.feedgenerator import Atom1Feed

    class RssSiteNewsFeed(Feed):
        title = "Chicagocrime.org site news"
        link = "/sitenews/"
        description = "Updates on changes and additions to chicagocrime.org."

        def items(self):
            return newsitems.get_list(order_by=('-pub_date',), limit=5)

    class AtomSiteNewsFeed(RssSiteNewsFeed):
        feed_type = Atom1Feed

And the accompanying URLconf::

    from django.conf.urls.defaults import *
    from myproject.feeds import RssSiteNewsFeed, AtomSiteNewsFeed

    feeds = {
        'rss': RssSiteNewsFeed,
        'atom': AtomSiteNewsFeed,
    }

    urlpatterns = patterns('',
        # ...
        (r'^feeds/(?P<url>.*)/$', 'django.contrib.syndication.views.feed',
            {'feed_dict': feeds}),
        # ...
    )

Feed class reference
--------------------

This example illustrates all possible attributes and methods for a ``Feed`` class::

    class ExampleFeed(rss.Feed):

        # FEED TYPE -- Optional. This should be a class that subclasses
        # django.utils.feedgenerator.SyndicationFeed. This designates which
        # type of feed this should be: RSS 2.0, Atom 1.0, etc.
        # If you don't specify feed_type, your feed will be RSS 2.0.
        # This should be a class, not an instance of the class.

        feed_type = feedgenerator.Rss201rev2Feed

        # TITLE -- One of the following three is required. The framework looks
        # for them in this order.

        def title(self, obj):
            """
            Takes the object returned by get_object() and returns the feed's
            title as a normal Python string.
            """

        def title(self):
            """
            Returns the feed's title as a normal Python string.
            """

        title = 'foo' # Hard-coded title.

        # LINK -- One of the following three is required. The framework looks
        # for them in this order.

        def link(self, obj):
            """
            Takes the object returned by get_object() and returns the feed's
            link as a normal Python string.
            """

        def link(self):
            """
            Returns the feed's link as a normal Python string.
            """

        link = '/foo/bar/' # Hard-coded link.

        # DESCRIPTION -- One of the following three is required. The framework
        # looks for them in this order.

        def description(self, obj):
            """
            Takes the object returned by get_object() and returns the feed's
            description as a normal Python string.
            """

        def description(self):
            """
            Returns the feed's description as a normal Python string.
            """

        description = 'Foo bar baz.' # Hard-coded description.

        # AUTHOR NAME --One of the following three is optional. The framework
        # looks for them in this order.

        def author_name(self, obj):
            """
            Takes the object returned by get_object() and returns the feed's
            author's name as a normal Python string.
            """

        def author_name(self):
            """
            Returns the feed's author's name as a normal Python string.
            """

        author_name = 'Sally Smith' # Hard-coded author name.

        # AUTHOR E-MAIL --One of the following three is optional. The framework
        # looks for them in this order.

        def author_email(self, obj):
            """
            Takes the object returned by get_object() and returns the feed's
            author's e-mail as a normal Python string.
            """

        def author_name(self):
            """
            Returns the feed's author's e-mail as a normal Python string.
            """

        author_email = 'test@example.com' # Hard-coded author e-mail.

        # AUTHOR LINK --One of the following three is optional. The framework
        # looks for them in this order. In each case, the URL should include
        # the "http://" and domain name.

        def author_link(self, obj):
            """
            Takes the object returned by get_object() and returns the feed's
            author's URL as a normal Python string.
            """

        def author_link(self):
            """
            Returns the feed's author's URL as a normal Python string.
            """

        author_link = 'http://www.example.com/' # Hard-coded author URL.

        # ITEMS -- One of the following three is required. The framework looks
        # for them in this order.

        def items(self, obj):
            """
            Takes the object returned by get_object() and returns a list of
            items to publish in this feed.
            """

        def items(self):
            """
            Returns a list of items to publish in this feed.
            """

        items = ('Item 1', 'Item 2') # Hard-coded items.

        # GET_OBJECT -- This is required for feeds that publish different data
        # for different URL parameters. (See "A complex example" above.)

        def get_object(self, bits):
            """
            Takes a list of strings gleaned from the URL and returns an object
            represented by this feed. Raises
            django.core.exceptions.ObjectDoesNotExist on error.
            """

        # ITEM LINK -- One of these three is required. The framework looks for
        # them in this order.

        # First, the framework tries the get_absolute_url() method on each item
        # returned by items(). Failing that, it tries these two methods:

        def item_link(self, item):
            """
            Takes an item, as returned by items(), and returns the item's URL.
            """

        def item_link(self):
            """
            Returns the URL for every item in the feed.
            """

        # ITEM AUTHOR NAME --One of the following three is optional. The
        # framework looks for them in this order.

        def item_author_name(self, item):
            """
            Takes an item, as returned by items(), and returns the item's
            author's name as a normal Python string.
            """

        def item_author_name(self):
            """
            Returns the author name for every item in the feed.
            """

        item_author_name = 'Sally Smith' # Hard-coded author name.

        # ITEM AUTHOR E-MAIL --One of the following three is optional. The
        # framework looks for them in this order.
        #
        # If you specify this, you must specify item_author_name.

        def item_author_email(self, obj):
            """
            Takes an item, as returned by items(), and returns the item's
            author's e-mail as a normal Python string.
            """

        def item_author_email(self):
            """
            Returns the author e-mail for every item in the feed.
            """

        item_author_email = 'test@example.com' # Hard-coded author e-mail.

        # ITEM AUTHOR LINK --One of the following three is optional. The
        # framework looks for them in this order. In each case, the URL should
        # include the "http://" and domain name.
        #
        # If you specify this, you must specify item_author_name.

        def item_author_link(self, obj):
            """
            Takes an item, as returned by items(), and returns the item's
            author's URL as a normal Python string.
            """

        def item_author_link(self):
            """
            Returns the author URL for every item in the feed.
            """

        item_author_link = 'http://www.example.com/' # Hard-coded author URL.

        # ITEM ENCLOSURE URL -- One of these three is required if you're
        # publishing enclosures. The framework looks for them in this order.

        def item_enclosure_url(self, item):
            """
            Takes an item, as returned by items(), and returns the item's
            enclosure URL.
            """

        def item_enclosure_url(self):
            """
            Returns the enclosure URL for every item in the feed.
            """

        item_enclosure_url = "/foo/bar.mp3" # Hard-coded enclosure link.

        # ITEM ENCLOSURE LENGTH -- One of these three is required if you're
        # publishing enclosures. The framework looks for them in this order.
        # In each case, the returned value should be either an integer, or a
        # string representation of the integer, in bytes.

        def item_enclosure_length(self, item):
            """
            Takes an item, as returned by items(), and returns the item's
            enclosure length.
            """

        def item_enclosure_length(self):
            """
            Returns the enclosure length for every item in the feed.
            """

        item_enclosure_length = 32000 # Hard-coded enclosure length.

        # ITEM ENCLOSURE MIME TYPE -- One of these three is required if you're
        # publishing enclosures. The framework looks for them in this order.

        def item_enclosure_mime_type(self, item):
            """
            Takes an item, as returned by items(), and returns the item's
            enclosure mime type.
            """

        def item_enclosure_mime_type(self):
            """
            Returns the enclosure length, in bytes, for every item in the feed.
            """

        item_enclosure_mime_type = "audio/mpeg" # Hard-coded enclosure mime-type.

        # ITEM PUBDATE -- It's optional to use one of these three. This is a
        # hook that specifies how to get the pubdate for a given item.
        # In each case, the method/attribute should return a Python
        # datetime.datetime object.

        def item_pubdate(self, item):
            """
            Takes an item, as returned by items(), and returns the item's
            pubdate.
            """

        def item_pubdate(self):
            """
            Returns the pubdate for every item in the feed.
            """

        item_pubdate = datetime.datetime(2005, 5, 3) # Hard-coded pubdate.

The low-level framework
=======================

Behind the scenes, the high-level RSS framework uses a lower-level framework
for generating feeds' XML. This framework lives in a single module:
`django/utils/feedgenerator.py`_.

Feel free to use this framework on your own, for lower-level tasks.

The ``feedgenerator`` module contains a base class ``SyndicationFeed`` and
several subclasses:

    * ``RssUserland091Feed``
    * ``Rss201rev2Feed``
    * ``Atom1Feed``

Each of these three classes knows how to render a certain type of feed as XML.
They share this interface:

``__init__(title, link, description, language=None, author_email=None,``
``author_name=None, author_link=None, subtitle=None, categories=None,``
``feed_url=None)``

Initializes the feed with the given metadata, which applies to the entire feed
(i.e., not just to a specific item in the feed).

All parameters, if given, should be Unicode objects, except ``categories``,
which should be a sequence of Unicode objects.

``add_item(title, link, description, author_email=None, author_name=None,``
``pubdate=None, comments=None, unique_id=None, enclosure=None, categories=())``

Add an item to the feed with the given parameters. All parameters, if given,
should be Unicode objects, except:

    * ``pubdate`` should be a `Python datetime object`_.
    * ``enclosure`` should be an instance of ``feedgenerator.Enclosure``.
    * ``categories`` should be a sequence of Unicode objects.

``write(outfile, encoding)``

Outputs the feed in the given encoding to outfile, which is a file-like object.

``writeString(encoding)``

Returns the feed as a string in the given encoding.

Example usage
-------------

This example creates an Atom 1.0 feed and prints it to standard output::

    >>> from django.utils import feedgenerator
    >>> f = feedgenerator.Atom1Feed(
    ...     title=u"My Weblog",
    ...     link=u"http://www.example.com/",
    ...     description=u"In which I write about what I ate today.",
    ...     language=u"en"),
    >>> f.add_item(title=u"Hot dog today",
    ...     link=u"http://www.example.com/entries/1/",
    ...     description=u"<p>Today I had a Vienna Beef hot dog. It was pink, plump and perfect.</p>")
    >>> print f.writeString('utf8')
    <?xml version="1.0" encoding="utf8"?>
    <feed xmlns="http://www.w3.org/2005/Atom" xml:lang="en"><title>My Weblog</title>
    <link href="http://www.example.com/"></link><id>http://www.example.com/</id>
    <updated>Sat, 12 Nov 2005 00:28:43 -0000</updated><entry><title>Hot dog today</title>
    <link>http://www.example.com/entries/1/</link><id>tag:www.example.com/entries/1/</id>
    <summary type="html">&lt;p&gt;Today I had a Vienna Beef hot dog. It was pink, plump and perfect.&lt;/p&gt;</summary>
    </entry></feed>

.. _django/utils/feedgenerator.py: http://code.djangoproject.com/browser/django/trunk/django/utils/feedgenerator.py
.. _Python datetime object: http://www.python.org/doc/current/lib/module-datetime.html