Allow CacheMiddleware.key_prefix to be a callable.

[Alexandru Mărășteanu]

As discussed in ​https://groups.google.com/g/django-developers/c/UbD1DkV1uPo and https://code.djangoproject.com/ticket/11269 it would be useful to be able to specify a callable which would allow for more dynamic caching. In my case, I need to cache differently depending on whether the user is authenticated or not. Another scenario is to cache certain pages depending on some timestamp. Other uses cases were mentioned in the linked pages.

There exists ​https://github.com/peterbe/django-fancy-cache but I was able to address this by extending the built-in middleware so I really don't see an argument for a 3rd party package.

(PR is 18254)​https://github.com/django/django/pull/18254

[Mariusz Felisiak]

Thanks for the ticket. Dynamic prefixes are not supported, however you can use the ​KEY_FUNCTION cache setting and changed key_prefix dynamically. I'm not convinced that we really need another option here.

[Alexandru Mărășteanu]

Replying to Mariusz Felisiak:

Thanks for the ticket. Dynamic prefixes are not supported, however you can use the ​KEY_FUNCTION cache setting and changed key_prefix dynamically. I'm not convinced that we really need another option here.

The need is to cache dynamically depending on the current request, on request properties like the user, or the query params etc. One of my use cases is to cache pages for anonymous users only. Another one is to cache pages based on some value stored in Redis. Another thing I want to do is to force it to skip the cache based on a query param (appending a random value is not always possible). Currently I do this with a custom middleware which extends the builtin one and simply updates self.key_prefix at the appropriate times. It seems to me it's a small change with great utility.

The KEY_FUNCTION would be useful here if it received the request. But allowing a callable key_prefix is a lot cheaper than changing the signature of KEY_FUNCTION.

[Mariusz Felisiak]

Tentatively accepted.

[Alexandru Mărășteanu]

Replying to Mariusz Felisiak:

Tentatively accepted.

I started writing some tests the other evening so I'll try to open a PR later today.

[Alexandru Mărășteanu]

I opened a PR at ​https://github.com/django/django/pull/15553 Unfortunately the tests stop running at some point due to an error which I've yet to figure out.

[Mariusz Felisiak]

Docs changes are missing. Also, many tests don't work.

[Alexandru Mărășteanu]

Replying to Mariusz Felisiak:

Docs changes are missing. Also, many tests don't work.

I fixed the tests and updated the docs