Content-negotiation for Django
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README.rst

Content-negotiation framework for Django

This project provides a simple and extensible framework for producing views that content-negotiate in Django.

Prerequisites

This library depends on Django 1.3, which you can install using your package manager on recent distributions, or using pip:

pip install -r requirements.txt

pip is called pip-python on Fedora. It is generally provided by a python-pip package.

Using

To define a view, do something like this:

from django_conneg.views import ContentNegotiatedView

class IndexView(ContentNegotiatedView):
    def get(self, request):
        context = {
            # Build context here
        }

        # Call render, passing a template name (without file extension)
        return self.render(request, context, 'index')

This will then look for a renderer that can provide a representation that matches what was asked for in the Accept header.

By default ContentNegotiatedView provides no renderers, so the above snippet would always return a 405 Not Acceptable to tell the user-agent that it couldn't provide a response in a suggested format.

To define a renderer on a view, do something like this:

import json

from django.http import HttpResponse

from django_conneg.decorators import renderer

class JSONView(ContentNegotiatedView):
    @renderer(format='json', mimetypes=('application/json',), name='JSON')
    def render_json(self, request, context, template_name):
        # Very simplistic, and will fail when it encounters 'non-primitives'
        # like Django Model objects, Forms, etc.
        return HttpResponse(json.dumps(context), mimetype='application/json')

Note

django-conneg already provides a slightly more sophisticated JSONView; see below for more information.

You can render to a particular format by calling render_to_format() on the view:

class IndexView(ContentNegotiatedView):
    def get(self, request):
        # ...

        if some_condition:
            return self.render_to_format(request, context, 'index', 'html')
        else:
            return self.render(request, context, 'index')

Forcing a particular renderer from the client

By default, a client can request a particular set of renderers be tried by using the format query or POST parameter:

GET /some-view/?format=json,yaml

The formats correspond to the format argument to the @renderer decorator.

To change the name of the parameter used, override _format_override_parameter on the view class:

class MyView(ContentNegotiatedView):
    _format_override_parameter = 'output'

Providing fallback renderers

Sometimes you might want to provide a response in some format even if the those in the Accept header can't be honoured. This is useful when providing error responses in a different format to the client's expected format. To do this, set the _force_fallback_format attribute to the name of the format:

class MyView(ContentNegotiatedView):
    _force_fallback_format = 'html'

If a client doesn't provide an Accept header, then you can specify a default format with _default_format:

class MyView(ContentNegotiatedView):
    _default_format = 'html'

Built-in renderer views

django_conneg includes the following built-in renderers in the django_conneg.views module:

  • HTMLView (renders a .html template with media type text/html)
  • TextView (renders a .txt template with media type text/plain)
  • JSONView (coerces the context to JavaScript primitives and returns as application/json)
  • JSONPView (as JSONView, but wraps in a callback and returns as application/javascript)

Using these, you could define a view that renders to both HTML and JSON like this:

from django_conneg.views import HTMLView

class IndexView(JSONView, HTMLView):
    def get(self, request):
        # ...
        return self.render(request, context, 'index')

Accessing renderer details

The renderer used to construct a response is exposed as a renderer attribute on the response object:

class IndexView(JSONView, HTMLView):
    def get(self, request):
        # ...
        response = self.render(request, context, 'index')
        response['X-Renderer-Format'] = response.renderer.format
        return response

Renderer priorities

Some user-agents might specify various media types with equal levels of desirability. For example, previous versions of Safari and Chrome used to send an Accept header like this:

application/xml,application/xhtml+xml,text/html;q=0.9,
text/plain;q=0.8,image/png,*/*;q=0.5

Without any additional hints it would be non-deterministic as to whether XML or XHTML is served.

By passing a priority argument to the @renderer decorator you can specify an ordering of renderers for such ambiguous situations:

class IndexView(ContentNegotiatedView):
    @renderer(format='xml', mimetypes=('application/xml',), name='XML', priority=0)
    def render_xml(request, context, template_name):
        # ...

    @renderer(format='html', mimetypes=('application/xhtml+xml','text/html), name='HTML', priority=1)
    def render_html(request, context, template_name):
        # ...

As higher-numbered priorities are preferred, this will result in HTML always being prefered over XML in ambiguous situations.

By default, django-conneg's built-in renderers have a priority of 0, except for HTMLView and TextView, which each have a priority of 1 for the reason given above.

Improved 40x response handling

Django provides a couple of useful exceptions, Http404 and PermissionDenied, which you may want to use in your application. However, it's only possible to customise the 404 site-wide (either by providing a 404.html template, or by setting handler404 in your urlconf), and until Django 1.4 comes out, PermissionDenied will always result in a very spartan error page.

django-conneg provides an ErrorCatchingView which you can use as a mixin to customise the rendering of responses for these error situations:

from django_conneg.views import HTMLView, ErrorCatchingView

class IndexView(HTMLView, ErrorCatchingView):
    # ...

You can then customise error responses in one of the following ways:

  • overriding the ``conneg/(forbidden|not_found|not_acceptable).(html|txt) templates
  • overriding error_403, error_404 or error_406 methods on the view
  • overriding the error_template_names attribute to specify a non-standard template name:

In the latter case, you can do something like:

import httplib
from django.util.datastructures import MergeDict
from django_conneg.views import HTMLView, ErrorCatchingView

class IndexView(HTMLView, ErrorCatchingView):
    # Provide a view-specific 404 page. Use MergeDict to use django_conneg's
    # defaults for other types of errors.
    error_template_names = MergeDict({httplib.NOT_FOUND: 'foo/404'},
                                     ErrorCatchingView.error_template_names)
    # ...

Running the tests

django-conneg has a modest test suite. To run it, head to the root of the repository and run:

django-admin test --settings=django_conneg.test_settings --pythonpath=.

If you don't have Django, you'll need to install it as detailed in the Prerequisites section above.