Refactor keyfunc from template cache tag

[nosa_manuel]

The code to generate keys is stuck in the middle of the render method of the cache tag node, and should be extracted so that it can be used to invalidate template cache fragments.

Patch available in a pull request here: ​https://github.com/django/django/pull/482

[Preston Holmes]

The only reason to refactor this is to expose it as public API - and so would need a slightly different approach.

The func should be exposed somewhere else - not in the template tag, perhaps in new django.core.cache.utils module

To avoid confusion with the make_key function on cache backends - it should be more explicitly named as make_templatefragment_key

And it should be documented at the end of ​https://docs.djangoproject.com/en/dev/topics/cache/#template-fragment-caching - presumably explaining the use case for, presumably, cache invalidation based on the change of something else in the project.

Could you explain why you can't achieve what you are after with using template variables for the vary-on args that are set by the view, which would cache a new fragment as needed?

[Tomek Paczkowski]

Added pull request: ​https://github.com/django/django/pull/818

[Honza Kral <honza.kral@…>]

In 99edbe0e279166db82caaf545ef92d5446a6a07e:

Fixed #19253 -- Extracted template cache key building logic

Introduced a public function
django.core.cache.utils.make_template_fragment_key
Thanks @chrismedrela for fruitful cooperation.

[Jannis Leidel]

I think this should have been put into django.utils.cache where other cache related utilities including cache key generation happen already.

Is there a particular reason why this was put into a completely new module instead?