Update decorator_from_middleware to work with new-style middleware

[Tim Graham]

@carljm is working on it: "I did work on updating decorator_from_middleware yesterday and have an in-progress patch. I don't think we should plan on the patch for alpha (it's nontrivial and will need review), but if it's ok to do it as a follow-up cleanup fix for beta, then yeah I think it's be good to have it for 1.10.

[Tim Graham]

Absent a compelling use case for this update and due to some limitations in how effectively decorator_from_middleware can make the transformation, we decided not to move forward with this. A query about it has been raised ​on Reddit. I've asked if that poster can give a use case here.

[Andreas Pelme]

A use case for this is tests. I have a middleware that handles temporary unavailability by showing a loading page/returning HTTP 503 for certain exceptions.

This particular middleware uses custom logic in __call__ and process_exception. decorator_from_middleware makes it easy to test this middleware:

@override_settings(MAINTENANCE_MODE=True)
def test_maintenance_mode(self):
    @decorator_from_middleware(TemporaryUnavailable)
    def view():
         raise AssertionError("don't call")
    response  = view(RequestFactory().get('/')
    assert response.status_code == 503

Of course, I could craft my own get_response, initiate the middleware and call __call__ or process_exception directly but that is ugly and easy to get some details wrong, especially when the test involves exceptions.

[Andreas Pelme]

Another use case:

We have a custom authentication mechanism that is not tied to contrib.admin for our main site.

However, we use contrib.auth for administrative accounts. To make it very clear which views needs a real user, we have disabled the auth middleware globally and use a custom admin site that selectively enables auth:

from django.contrib import admin
from django.contrib.auth.middleware import AuthenticationMiddleware

from django.utils.decorators import decorator_from_middleware


auth_decorator = decorator_from_middleware(AuthenticationMiddleware)


class AdminSite(admin.AdminSite):
    def admin_view(self, view, cacheable=False):
        super_wrapper = super().admin_view(view, cacheable=cacheable)
        return auth_decorator(super_wrapper)

(This use case is fine and works fine in Django 1.10 since Django's built in AuthenticationMiddleware is both old and new-style, but I just wanted to highlight that decorator_from_middleware is useful in different contexts)

[David Svenson]

The following function takes a new-style middleware class and makes a view decorator out of it. It's used in code that me and @pelme are working on, though not as a decorator.

def decorator_from_middleware_new(new_middleware_cls):
    def view_decorator(view_function):
        def view(request, *args, **kwargs):
            def get_response(request_inner):
                assert request is request_inner
    
                try:
                    return view_function(request, *args, **kwargs)
                except Exception as e:
                    new_middleware.process_exception(request, e)
                    return HttpResponseServerError()

            new_middleware = new_middleware_cls(get_response)
            return new_middleware(request)

        return view

    return view_decorator