The Django Book

Installing ReportLab

Before you do any PDF generation, however, you’ll need to install ReportLab. It’s usually pretty simple: just download and install the library from http://www.reportlab.org/downloads.html.

The user guide (naturally available only as a PDF file) at http://www.reportlab.org/rsrc/userguide.pdf has additional installation instructions.

Test your installation by importing it in the Python interactive interpreter:

>>> import reportlab

If that command doesn’t raise any errors, the installation worked.

Writing Your View

Like CSV, generating PDFs dynamically with Django is easy because the ReportLab API acts on filelike objects.

Here’s a “Hello World” example:

from reportlab.pdfgen import canvas
from django.http import HttpResponse

def hello_pdf(request):
    # Create the HttpResponse object with the appropriate PDF headers.
    response = HttpResponse(mimetype='application/pdf')
    response['Content-Disposition'] = 'attachment; filename=hello.pdf'

    # Create the PDF object, using the response object as its "file."
    p = canvas.Canvas(response)

    # Draw things on the PDF. Here's where the PDF generation happens.
    # See the ReportLab documentation for the full list of functionality.
    p.drawString(100, 100, "Hello world.")

    # Close the PDF object cleanly, and we're done.
    p.showPage()
    p.save()
    return response

A few notes are in order:

• Here we use the application/pdf MIME type. This tells browsers that the document is a PDF file, rather than an HTML file. If you leave off this information, browsers will probably interpret the response as HTML, which will result in scary gobbledygook in the browser window.
• Hooking into the ReportLab API is easy: just pass response as the first argument to canvas.Canvas. The Canvas class expects a filelike object, and HttpResponse objects fit the bill.
• All subsequent PDF-generation methods are called on the PDF object (in this case, p), not on response.
• Finally, it’s important to call showPage() and save() on the PDF file (or else you’ll end up with a corrupted PDF file).

Complex PDFs

If you’re creating a complex PDF document (or any large data blob), consider using the cStringIO library as a temporary holding place for your PDF file. The cStringIO library provides a file-like object interface that is written in C for maximum efficiency.

Here’s the previous “Hello World” example rewritten to use cStringIO:

from cStringIO import StringIO
from reportlab.pdfgen import canvas
from django.http import HttpResponse

def hello_pdf(request):
    # Create the HttpResponse object with the appropriate PDF headers.
    response = HttpResponse(mimetype='application/pdf')
    response['Content-Disposition'] = 'attachment; filename=hello.pdf'

    temp = StringIO()

    # Create the PDF object, using the StringIO object as its "file."
    p = canvas.Canvas(temp)

    # Draw things on the PDF. Here's where the PDF generation happens.
    # See the ReportLab documentation for the full list of functionality.
    p.drawString(100, 100, "Hello world.")

    # Close the PDF object cleanly.
    p.showPage()
    p.save()

    # Get the value of the StringIO buffer and write it to the response.
    response.write(temp.getvalue())
    return response

Initialization

To activate syndication feeds on your Django site, add this URLconf:

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

This line 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 preceding 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’ll need to define the Feed classes themselves.

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 code tree.

A Simple Feed

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

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

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

    def items(self):
        return NewsItem.objects.order_by('-pub_date')[:5]

The important things to notice here are as follows:

• 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 database API, items() doesn’t have to return model instances.

  You do get a few bits of functionality “for free” by using Django models, but items() can return any type of object you want.

There’s just one more step. 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 (see Chapter 4) called feeds/latest_title.html and feeds/latest_description.html, where latest is the slug specified in the URLconf for the given feed. Note that the .html extension is required.

  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.

  You can also change the names of these two templates by specifying title_template and description_template as attributes of your Feed class.

• 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.

• For the previous LatestEntries example, we could have very simple feed templates. latest_title.html contains:

  {{ obj.title }}
  

  and latest_description.html contains:

  {{ obj.description }}
  

  It’s almost too easy …

A More Complex Feed

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 would be silly to create a separate Feed class for each police beat; that would violate the Don’t Repeat Yourself (DRY) principle and would couple data to programming logic.

Instead, the syndication framework lets you make generic feeds that return items based on information in the feed’s URL.

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

• http://www.chicagocrime.org/rss/beats/0613/: Returns recent crimes for beat 0613
• http://www.chicagocrime.org/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:

from django.core.exceptions import ObjectDoesNotExist

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 Beat.objects.get(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):
        crimes =  Crime.objects.filter(beat__id__exact=obj.id)
        return crimes.order_by('-crime_date')[:30]

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

1. 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’s 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'].

2. 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 Beat.objects.get() call, because it’s not necessary. That function raises Beat.DoesNotExist on failure, and Beat.DoesNotExist is a subclass of ObjectDoesNotExist. Raising ObjectDoesNotExist in get_object() tells Django to produce a 404 error for that request.

3. 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:

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

4. 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(), and then finally an items class attribute (which should be a list).

Full documentation of all the methods and attributes of the Feed classes is always available from the official Django documentation (http://www.djangoproject.com/documentation/0.96/syndication_feeds/).

Specifying the Type of Feed

By default, the syndication framework produces RSS 2.0. To change that, add a feed_type attribute to your Feed class:

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 shown in Table 11-1.

Enclosures

To specify enclosures (i.e., media resources associated with a feed item such as MP3 podcast feeds), use the item_enclosure_url, item_enclosure_length, and item_enclosure_mime_type hooks, for example:

from myproject.models import Song

class MyFeedWithEnclosures(Feed):
    title = "Example feed with enclosures"
    link = "/feeds/example-with-enclosures/"

    def items(self):
        return Song.objects.all()[:30]

    def item_enclosure_url(self, item):
        return item.song_url

    def item_enclosure_length(self, item):
        return item.song_length

    item_enclosure_mime_type = "audio/mpeg"

This assumes, of course, that you’ve created a Song object with song_url and song_length (i.e., the size in bytes) fields.

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.

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.

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 chicagocrime.models import NewsItem
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 NewsItem.objects.order_by('-pub_date')[:5]

class AtomSiteNewsFeed(RssSiteNewsFeed):
    feed_type = Atom1Feed

And here’s 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}),
    # ...
)

Installation

To install the sitemap application, follow these steps:

1. Add 'django.contrib.sitemaps' to your INSTALLED_APPS setting.
2. Make sure 'django.template.loaders.app_directories.load_template_source' is in your TEMPLATE_LOADERS setting. It’s in there by default, so you’ll need to change this only if you’ve changed that setting.
3. Make sure you’ve installed the sites framework (see Chapter 14).

Initialization

To activate sitemap generation on your Django site, add this line to your URLconf:

(r'^sitemap.xml$', 'django.contrib.sitemaps.views.sitemap', {'sitemaps': sitemaps})

This line tells Django to build a sitemap when a client accesses /sitemap.xml.

The name of the sitemap file is not important, but the location is. Search engines will only index links in your sitemap for the current URL level and below. For instance, if sitemap.xml lives in your root directory, it may reference any URL in your site. However, if your sitemap lives at /content/sitemap.xml, it may only reference URLs that begin with /content/.

The sitemap view takes an extra, required argument: {'sitemaps': sitemaps}. sitemaps should be a dictionary that maps a short section label (e.g., blog or news) to its Sitemap class (e.g., BlogSitemap or NewsSitemap). It may also map to an instance of a Sitemap class (e.g., BlogSitemap(some_var)).

Sitemap Classes

A Sitemap class is a simple Python class that represents a “section” of entries in your sitemap. For example, one Sitemap class could represent all the entries of your Weblog, while another could represent all of the events in your events calendar.

In the simplest case, all these sections get lumped together into one sitemap.xml, but it’s also possible to use the framework to generate a sitemap index that references individual sitemap files, one per section (as described shortly).

Sitemap classes must subclass django.contrib.sitemaps.Sitemap. They can live anywhere in your code tree.

For example, let’s assume you have a blog system, with an Entry model, and you want your sitemap to include all the links to your individual blog entries. Here’s how your Sitemap class might look:

from django.contrib.sitemaps import Sitemap
from mysite.blog.models import Entry

class BlogSitemap(Sitemap):
    changefreq = "never"
    priority = 0.5

    def items(self):
        return Entry.objects.filter(is_draft=False)

    def lastmod(self, obj):
        return obj.pub_date

Declaring a Sitemap should look very similar to declaring a Feed; that’s by design.

Like Feed classes, Sitemap members can be either methods or attributes. See the steps in the earlier “A Complex Example” section for more about how this works.

A Sitemap class can define the following methods/attributes:

• items (required): Provides list of objects. The framework doesn’t care what type of objects they are; all that matters is that these objects get passed to the location(), lastmod(), changefreq(), and priority() methods.

• location (optional): Gives the absolute URL for a given object. Here, “absolute URL” means a URL that doesn’t include the protocol or domain. Here are some examples:

  • Good: '/foo/bar/'
  • Bad: 'example.com/foo/bar/'
  • Bad: 'http://example.com/foo/bar/'

  If location isn’t provided, the framework will call the get_absolute_url() method on each object as returned by items().

• lastmod (optional): The object’s “last modification” date, as a Python datetime object.

• changefreq (optional): How often the object changes. Possible values (as given by the Sitemaps specification) are as follows:

  • 'always'
  • 'hourly'
  • 'daily'
  • 'weekly'
  • 'monthly'
  • 'yearly'
  • 'never'

• priority (optional): A suggested indexing priority between 0.0 and 1.0. The default priority of a page is 0.5; see the http://sitemaps.org documentation for more about how priority works.

Shortcuts

The sitemap framework provides a couple convenience classes for common cases. These are described in the sections that follow.

from django.conf.urls.defaults import *
from django.contrib.sitemaps import FlatPageSitemap, GenericSitemap
from mysite.blog.models import Entry

info_dict = {
    'queryset': Entry.objects.all(),
    'date_field': 'pub_date',
}

sitemaps = {
    'flatpages': FlatPageSitemap,
    'blog': GenericSitemap(info_dict, priority=0.6),
}

urlpatterns = patterns('',
    # some generic view using info_dict
    # ...

    # the sitemap
    (r'^sitemap.xml$',
     'django.contrib.sitemaps.views.sitemap',
     {'sitemaps': sitemaps})
)

Creating a Sitemap Index

The sitemap framework also has the ability to create a sitemap index that references individual sitemap files, one per each section defined in your sitemaps dictionary. The only differences in usage are as follows:

• You use two views in your URLconf: django.contrib.sitemaps.views.index and django.contrib.sitemaps.views.sitemap.
• The django.contrib.sitemaps.views.sitemap view should take a section keyword argument.

Here is what the relevant URLconf lines would look like for the previous example:

(r'^sitemap.xml$',
 'django.contrib.sitemaps.views.index',
 {'sitemaps': sitemaps}),

(r'^sitemap-(?P<section>.+).xml$',
 'django.contrib.sitemaps.views.sitemap',
 {'sitemaps': sitemaps})

This will automatically generate a sitemap.xml file that references both sitemap-flatpages.xml and sitemap-blog.xml. The Sitemap classes and the sitemaps dictionary don’t change at all.

Pinging Google

You may want to “ping” Google when your sitemap changes, to let it know to reindex your site. The framework provides a function to do just that: django.contrib.sitemaps.ping_google().

ping_google() takes an optional argument, sitemap_url, which should be the absolute URL of your site’s sitemap (e.g., '/sitemap.xml'). If this argument isn’t provided, ping_google() will attempt to figure out your sitemap by performing a reverse lookup on your URLconf.

ping_google() raises the exception django.contrib.sitemaps.SitemapNotFound if it cannot determine your sitemap URL.

One useful way to call ping_google() is from a model’s save() method:

from django.contrib.sitemaps import ping_google

class Entry(models.Model):
    # ...
    def save(self):
        super(Entry, self).save()
        try:
            ping_google()
        except Exception:
            # Bare 'except' because we could get a variety
            # of HTTP-related exceptions.
            pass

A more efficient solution, however, would be to call ping_google() from a cron script or some other scheduled task. The function makes an HTTP request to Google’s servers, so you may not want to introduce that network overhead each time you call save().