Previously we used None as the sentinel for determining that a user had
not specified a default value for a field. This meant that a user could
never select `None` as the default value. We now properly use UNSET to
determine if a default was passed, this aligns better with the code as
it stands given that if no default is passed we'll use UNSET anyway.
This commit is a fairly large chunk of code that fixes a previously
annoying and nasty bug causing cached_properties to not be cleared.
Alongside this bug fix I took the opportunity to refactor the entire
methdology behind cached properties, and bind them more strictly to the
behavior of models.
Prior to this commit, cached_properties where not be properly reset upon
a model's `update` method being called due to some invalid code. Along
with this issue, the actual behavior of cached properties landed in a
weird in-between/gray area where they had to support both non-model and
model use cases.
The new version of cached_property is strictly for use with models, and
another version `simple_cached_property` was added to support other use
cases. The new cached_property simply builds an actual `property` within
the metaclass. This is fairly efficient, and also reduces the surface
area of behavior.
When writing this I messed around with a few ideas, including allowing
the user of `cached_property` to define a linkage between the property
and fields that should invalidate its cached value, but this was both
messy and introduced massive cognitive load on understand when a
cached_property should be cleared. Although this version is slighty less
efficient, I'm very much in favor of it vs the alternatives I tried.
andrei