An idea for improving the template fragment cache

[HongWeipeng]

There is a problem in using template fragment caching, that is, we need transfer data for template every time, even the data has been in cache.

If the data acquisition process is time consuming (such as 3 seconds), template caching has no helpful.

My idea is to allow provide a callback function to tell the cache how to get the data. and only call the function when cache expires.

I have tried it many times and found out how to make the simplest changes to complete this feature and ensure its compatibility.

Can you see this RP please? ​https://github.com/django/django/pull/10176