Middlewares load twice due to StaticFilesHandler(WSGIHandler)

[Anthony King]

In Django 1.10, a new middleware system was implemented. As part of this implementation, loading of middleware was moved to WSGIHandler.init.

When django.contrib.staticfiles is used, StaticFilesHandler is used as part of runserver, which causes WSGIHandler.__init__ to be called twice.

This, in turn, causes the middlewares to all be initialised twice.

This will affect the majority of development environments. It should not affect production environments, as people should not be using runserver.

With the example middleware, you can see the class being initialised twice.

# middleware.py
class ExampleMiddleware(MiddlewareMixin):

    def __init__(self, get_response=None):
        super().__init__(get_response=get_response)
        print("simple_middleware - class: ", time.time(), self)

[Anthony King]

introduced in ​https://github.com/django/django/commit/99bb7fcc1859615a7b8c2468e7b97d54853bfb10

[Carl Meyer]

I've always thought it a bit strange that StaticFilesHandler inherits WSGIHandler; it seems prone to unintended effects like this one. IMO the right fix here is to make StaticFilesHandler self-contained, even if that requires a bit more verbose WSGI-handling code in it.

[Patrick Nicholls]

Are you still working on this Chandrakant Kumar? If not I may give it a crack.

[Claude Paroz]

I was not sure about inheritance drop and as the fix was rather straightforward, I'm still proposing a ​PR which keeps WSGIHandler inheritance.

[Claude Paroz <claude@…>]

In 09b3e466:

Fixed #27796 -- Prevented middleware being loaded twice with runserver

Thanks Tim Graham for the review.