#30864: Document and endorse django.utils.decorators.classproperty
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Reporter: jgonggrijp | Owner: nobody
Type: New feature | Status: new
Component: Documentation | Version: 2.2
Severity: Normal | Resolution:
Keywords: utils classproperty | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
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Comment (by Simon Charette):
> Personally, I'd love having `cached_classproperty`…
That would be hard to implement without some metaclass mockery or relying
on some naive `if not hasattr(cls, '_cached_foo')` constructs.
`cached_property` is efficient because it relies on the descriptor
protocol to cache its return value into `__dict__` and avoid class level
attribute lookups. We can't achieve similar things without defining the
attribute on the class of the class (metaclass) and use the class's
`__dict__` as a cache store.
I guess the following could work.
{{{#!python
class cached_classproperty:
NOT_SET = object()
def __init__(self, method):
self.method = method
self.cached_value = self.NOT_SET
def __get__(self, instance, owner):
if self.cached_value is not self.NOT_SET:
return self.cached_value
self.cached_value = self.method(owner)
return self.cached_value
def __set__(self, instance, value):
self.cached_value = value
def __delete__(self, instance):
self.cached_value = self.NOT_SET
}}}
But this wouldn't be as efficient as `cached_property` because `__get__`
would always be called.
Anyway the `cached_classproperty` case should probably be discussed in
another ticket or on the mailing list.
--
Ticket URL: <https://code.djangoproject.com/ticket/30864#comment:2>
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