Class views in Django

Question

Django view points to a function, which can be a problem if you want to change only a bit of functionality. Yes, I could have million keyword arguments and even more if statements in the function, but I was thinking more of an object oriented approach.

For example, I have a page that displays a user. This page is very similar to page that displays a group, but it's still not so similar to just use another data model. Group also has members etc...

One way would be to point views to class methods and then extend that class. Has anyone tried this approach or has any other idea?

Solution 1

I've created and used my own generic view classes, defining __call__ so an instance of the class is callable. I really like it; while Django's generic views allow some customization through keyword arguments, OO generic views (if their behavior is split into a number of separate methods) can have much more fine-grained customization via subclassing, which lets me repeat myself a lot less. (I get tired of rewriting the same create/update view logic anytime I need to tweak something Django's generic views don't quite allow).

I've posted some code at djangosnippets.org.

The only real downside I see is the proliferation of internal method calls, which may impact performance somewhat. I don't think this is much of a concern; it's rare that Python code execution would be your performance bottleneck in a web app.

UPDATE: Django's own generic views are now class-based.

UPDATE: FWIW, I've changed my opinion on class-based views since this answer was written. After having used them extensively on a couple of projects, I feel they tend to lead to code that is satisfyingly DRY to write, but very hard to read and maintain later, because functionality is spread across so many different places, and subclasses are so dependent on every implementation detail of the superclasses and mixins. I now feel that TemplateResponse and view decorators is a better answer for decomposing view code.

Solution 2

I needed to use class based views, but I wanted to be able to use the full name of the class in my URLconf without always having to instantiate the view class before using it. What helped me was a surprisingly simple metaclass:

class CallableViewClass(type):
    def __call__(cls, *args, **kwargs):
        if args and isinstance(args[0], HttpRequest):
            instance = super(CallableViewClass, cls).__call__()
            return instance.__call__(*args, **kwargs)
        else:
            instance = super(CallableViewClass, cls).__call__(*args, **kwargs)
            return instance


class View(object):
    __metaclass__ = CallableViewClass

    def __call__(self, request, *args, **kwargs):
        if hasattr(self, request.method):
            handler = getattr(self, request.method)
            if hasattr(handler, '__call__'):
                return handler(request, *args, **kwargs)
        return HttpResponseBadRequest('Method Not Allowed', status=405)

I can now both instantiate view classes and use the instances as view functions, OR I can simply point my URLconf to my class and have the metaclass instantiate (and call) the view class for me. This works by checking the first argument to __call__ – if it's a HttpRequest, it must be an actual HTTP request because it would be nonsense to attept to instantiate a view class with an HttpRequest instance.

class MyView(View):
    def __init__(self, arg=None):
        self.arg = arg
    def GET(request):
        return HttpResponse(self.arg or 'no args provided')

@login_required
class MyOtherView(View):
    def POST(request):
        pass

# And all the following work as expected.
urlpatterns = patterns(''
    url(r'^myview1$', 'myapp.views.MyView', name='myview1'),
    url(r'^myview2$', myapp.views.MyView, name='myview2'),
    url(r'^myview3$', myapp.views.MyView('foobar'), name='myview3'),
    url(r'^myotherview$', 'myapp.views.MyOtherView', name='otherview'),
)

(I posted a snippet for this at http://djangosnippets.org/snippets/2041/)

Solution 3

If you're simply displaying data from models, why not use the Django Generic Views? They're designed to let you easy show data from a model without having to write your own view and stuff about mapping URL paramaters to views, fetching data, handling edge cases, rendering output, etc.

Solution 4

You can always create a class, override the __call__ function and then point the URL file to an instance of the class. You can take a look at the FormWizard class to see how this is done.

Solution 5

Unless you want to do something a little complex, using the generic views are the way to go. They are far more powerful than their name implies, and if you are just displaying model data generic views will do the job.

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Comments

[user1066101]

The markup processor ate your __'s. Use \_ to hide the markup. The class __call__ method is what's called. The issue is to be sure that the urls.py has an instance of the class available to it. It makes your urls.py somewhat more complicated.

[Attila O.]

I don't think putting too much logic in the template is a good idea.

[StefanNch]

Python execution is not THE bottleneck, but could affect the site scalability, and overall the performance. I'm so glad memcache exists!