Hi all, Currently, middleware is initialized lazily on serving the first request, rather than on application start. There may well have been good reasons for this historically, but I don't think they apply any longer. Lazy initialization is unhelpful if a middleware class throws an error (e.g to report a misconfiguration) because the application will appear to start successfully and only later report the error when a request is made.
I'd like to propose initializing middleware when `get_wsgi_application` is called. This solves the problem described above and, as far as I can tell, raises no backwards compatibility issues. More details on all this below. ### 1. Specific example of the problem I recently wrote an adapter for the WhiteNoise static file server so it could function as Django middleware as well as WSGI middleware (https://github.com/evansd/whitenoise). WhiteNoise may be unusual in doing a non-trivial amount of work on initialization, but it doesn't seem unreasonable. When used as WSGI middleware any errors are triggered immediately on start up, but not so when used as Django middleware. This makes for a worse developer experience and an increased chance of deployment errors. ### 2. Reasons previously given for lazy initialization There was some brief discussion in this ticket 4 years ago: https://code.djangoproject.com/ticket/18577 The reason given there is that "resolving on first request makes most sense, especially for the case where you might not be serving requests at all". Presumably this refers to running non-http-related management commands. But in those cases we never instantiate a WSGI application anyway (wsgi.py is just never imported) so this is no reason not to initialize eagerly when constructing the WSGI application. (Of course, things may have been different 4 years ago.) Another reason is given in the comments in django.core.handles.wsgi: https://github.com/django/django/blob/3c1b572f1815c295878795b183b1957d0df2ca39/django/core/handlers/wsgi.py#L154 This says "Set up middleware if needed. We couldn't do this earlier, because settings weren't available". However `get_wsgi_application` (the only public WSGI API) now calls `django.setup()` before constructing the handler so settings are in fact available. ### 3. Proposed solution My proposal is simply to amend `get_wsgi_application` as follows: def get_wsgi_application(): django.setup(set_prefix=False) handler = WSGIHandler() handler.load_middleware() return handler It's possible that this logic could be moved into the handler's __init__ method. This caused no problems with existing application when I tried it, however it did cause problems with the test suite which seems to rely on the old behaviour in places. The above proposal passes all existing tests as is. ### 4. Backwards compatibility issues Middleware constructors have no means of accessing the request object or anything that depends on it. They are called right at the start of the handler's `__call__` method before the `request_started` signal is sent and before the `script_prefix` thread-local is set. Therefore it cannot matter, from the middleware class's perspective, whether it is instantiated before or after the first request comes in. I'm aware this issue probably isn't high on anyone else's priority list, but I think it would count as a genuine -- if small -- improvement to Django. Thanks, Dave -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-developers+unsubscr...@googlegroups.com. To post to this group, send email to django-developers@googlegroups.com. Visit this group at https://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/a8b2fc46-5bc3-4797-9e94-052eb3e35e8a%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
