Using WeakValueDictionary as a cachestore

[vaab]

This is a copy of Thomas Kemmer email suggestion::

A last one for today: I only recently came across

https://docs.python.org/3/library/weakref.html

I think WeakValueDictionary makes for a very interesting "standard" cache-like class. Maybe you don't want to replace good old dict as your default, but you should definitely check your decorator works with it, and maybe say so in the docs. Since it's quite a new thing, even seasoned Python programmers may not have heard about it.

[rouilj]

Just to let you know I have tested kids.cache with WeakValueDictionary. It works fine
for my use cases. Are you still interested in pull requests for this code?

I have 4 patches for my use cases that I can open tickets on or generate pull requests if you are still maintaining this.

The 4 patches are:

• Bypass - allow the function that determines the key to skip the cache and always run the decorated function
• Contents - allow the user to access the cache storage for debugging (or possible modification danger)
• Maxsize - the default dict storage grows without bound. This adds a maxsize option and cache_trim() method to control memory use. Using a cache from the python cachetools module is better, but brings in a dependency. This addresses an issue on the main site.
• WeakValueDictionary - This allows a WeakValueDictionary to be used as a storage option with maxsize support. It also documents how to use a WeakValueDictionary with kids.cache. This addresses an issue on the upstream site. (aka this issue)

I have code for the Readme that demonstrates WeakValueDictionary use.
Also I think #10 might be caused by the cache holding onto objects and preventing the objects from actually being deleted.
In that case using a WeakValueDictionary should be the solution.