= Class-Based Views = 

The proposal to add a class-based framework for describing views (#6735) has been around for a while. It's been proposed for inclusion in Django 1.0, 1.1, and 1.2. This is a summary of the state of debate, with the intention of landing a patch for the 1.3 release.

== The brief ==

At present, generic views are defined as methods. If you want to customize them, you have to pass in arguments to the view at time of use in urls.py. This is cumbersome:

 * urls.py rapidly becomes large an unwieldly
 * complex behaviors that need to be wrapped in callables depend on the underlying views handling callable arguments (which isn't implemented universally)
 * There's no such thing as a simple extension -- you can't easily say "use that view, but change one argument"; you have to reproduce the full argument list.
 * .. and much more 

Moving to a class-based structure means that the complexities of defining and customizing generic views can be handled by subclassing. urls.py returns to being a simple process of pointing at a class. Extension logic can be arbitrarily complex, abstracted behind methods on the generic class. It also means we can provide an entry point for HTTP method-based dispatch -- e.g., a GET() method to handle GET requests, POST() to handle posts, etc.

== The problem ==

However, the devil is in the detail. There are several issues that any class-based view solution needs to address:

 * '''Deployment''': How do you deploy an instance of the view
 * '''URLResolver interaction''': Does the approach require any special handling in URLResolver to identify and/or instantiate the view?
 * '''Subclassing''': How easy is it to subclass and override a specific method on a view class?
 * '''Thread safety''': Does each request get a clean instance of self to work with? If so, how is this achieved?
 * '''Ease of testing''': Does testing the view require any special handling (such as a specialized method to instantiate the view)
 * '''Ease of decoration''': Does the proposed technique pose any impediments to easily decorating views?

The [http://groups.google.com/group/django-developers/browse_thread/thread/e23d9a5a58fe5891 most recent django-developers discussion on the topic] discussed many of the available options. Here is a summary of the various approaches that have been proposed.

=== !__call!__() and copy() ===

[http://github.com/bfirsh/django-class-based-views Implementation]

Example usage:
{{{
    url(r'^detail/author/(?P<pk>\d+)/$', views.AuthorDetail(), name="author_detail"),
}}}

Example class:
{{{
class AuthorView(View):
    def GET(self, request, *args, **kwargs)
        return self.render_to_response('author_list.html', {'authors': Author.objects.all()})
}}}

This approach proposes that an class instance be placed in urls.py; the instance has a !__call!__() method, and when that method is invoked, it takes a shallow copy of the instance defined in urls.py, and returns the result of invoking the request method (e.g., GET()). This achieves thread safety by ensuring that every request is given a clean instance on the view class to work on.

No special handling is required in !UrlResolver -- the class instance is a callable, so it appears just like an existing view function.

Arguments against:
 * The "copy on !__call!__()" approach is a little messy. Normal Python idiom wouldn't lead users to expect that !__call!__() would cause a copy to be created. However, this is entirely hidden by the implementation.

=== !__new!__() === 

[http://github.com/fitzgen/django-class-based-views Implementation]

Example usage:
{{{
    url(r'^detail/author/(?P<pk>\d+)/$', views.AuthorDetail, name="author_detail"),
}}}

Example class:
{{{
class AuthorView(View):
    def GET(self, request, *args, **kwargs)
        return self.render_to_response('author_list.html', {'authors': Author.objects.all()})
}}}

This approach is much the same as the !__copy!__() on !__call!__() approach, except that !__new!__() is used to create the new instance.

Arguments against:
 * You can't use arguments to __init__() to instantiate a class view
 * x = !AuthorView() returns x as a HTTPResponse, not an !AuthorView instance. This violates expectations of normal class usage.

=== HTTPResponse subclassing ===

[http://github.com/roalddevries/class_based_views/ Implementation]

This approach exploits the fact that the aim of a view is to produce a HttpResponse instance; so it shortcuts the process, and makes a 'view' the constructor for the !HttpResponse.

Arguments against:
 * Binds the Response object the concept of a view. A view isn't 'is-a' response, in the OO-sense.
 * Makes it difficult or impossible to subclass !HttpResponse and use that subclass
 * Makes it difficult to use !HttpResponse-returning methods; you can't just call out to a subview/utility method that returns a !HttpResponse instance, because the view needs to return 'self'.

=== !UrlResolver view instantiation ===

Rather than try to make the view look like a callable, this approach modifies !UrlResolver so that it can identify when a class-based view is in use, and instantiate an instance when one is detected.

Arguments against:
 * Requires special cases in !UrlResolver which almost inevitably involve isinstance(!ViewClass) or some analog.

= Recommendation =

Based on these discussions, plus in-person discussions at !DjangoCon.eu,  !__copy!__() on !__call!__() appears to be a slight front runner, with !__new!__() as a close runner up.