Are Instance variables of class-based-views persistent?

Question

I've just started using class-based views in django. But there's an issue that is confusing for me. I ran the following code snippet with django 1.4.1 with multithreaded development server.

class TestView(TemplateView):
    template_name = 'test.html'
    count = 0
    mylist = [1, ]

    def get(self, request, *args, **kwargs):
        self.count += 1
        self.mylist.append(self.mylist[-1] +1)
        context = self.get_context_data(**kwargs)
        return self.render_to_response(context)

    def get_context_data(self, **kwargs):
        context = super(TestView, self).get_context_data(**kwargs)
        context['count'] = self.count
        context['mylist'] = self.mylist
        return context

The template just outputs the context variables count and mylist. When this view is called e.g. up to 5 times the output will look like this:

count: 1
mylist: [1, 2, 3, 4, 5, ]

And now I'm confused. The django docs says, that each request has its own individual class instance.

So how it is possible to extend mylist over several requests? Why the count variable was not incremented?

Answer 1

If you need something available over several requests, you need to add it to the session. An ivar is not the place for that. For thread-safety, the as_view method returns a brand new instance of the class each time, which is not effected by anything else that happens on any other instances of the same class. This is by design and you'd have numerous problems if it wasn't the case.