Document and endorse django.utils.decorators.classproperty

[Julian Gonggrijp]

django.utils.decorators.classproperty is really useful. Please document it, encourage its use and make it a stable feature.

[Claude Paroz]

Personally, I'd love having cached_classproperty…

[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.

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.

[Carlton Gibson]

I don't see any harm in promoting this to public.

[Mariusz Felisiak]

I can only add that it's already tested utils_tests.test_decorators.ClassPropertyTest. Documentation should land in docs/ref/utils.txt.

[Claude Paroz]

Shouldn't it be moved to django.utils.functional?

[Mariusz Felisiak]

Yes, good idea.

[André Ericson]

Replying to Claude Paroz:

Shouldn't it be moved to django.utils.functional?

I've created a ticket for that https://code.djangoproject.com/ticket/30876 I think it makes more sense to address it first. I will go ahead and do it once it's accepted.

[Deep Sukhwani]

​PR#12637

[Deep Sukhwani]

For reviewers: Slightly confused since this is my first trac ticket in Django project. Please guide.

Three questions:

• The version on this ticket says 2.2 however the change, i.e. move from django.utils.decorators to django.utils.functional was made in Django 3.1 (current ongoing development version) as a result of which, I added versionchanged text to the submitted code change. Should I remove versionchanged?
• If the answer for above question is yes, does this mean this change needs to be backported to 2.2?
• If the answer for above question is yes, then does this also need to be backported to every other version release since 2.2? i.e. all Versions: in 2.x.y series and versions in 3.0.x series?

[Mariusz Felisiak]

Replying to Deep Sukhwani:

• The version on this ticket says 2.2 however the change, i.e. move from django.utils.decorators to django.utils.functional was made in Django 3.1 (current ongoing development version) as a result of which, I added versionchanged text to the submitted code change. Should I remove versionchanged?

This is a new feature so it goes only to the current master and should be targeted to the Django 3.1. versionchanged annotation is not necessary, you should add versionadded instead.

[Deep Sukhwani]

Replying to felixxm:

This is a new feature so it goes only to the current master and should be targeted to the Django 3.1. versionchanged annotation is not necessary, you should add versionadded instead.

This feature (classproperty decorator) did exist in previous version, however it was part of a different module django.utils.decorators and in version 3.1 it is moved to django.utils.functional

Ref: ​https://docs.djangoproject.com/en/dev/releases/3.1/#id1, it says

django.utils.decorators.classproperty() decorator is moved to django.utils.functional.classproperty()

[Mariusz Felisiak]

I know that, but it doesn't change anything, IMO. We've added this note in case someone uses this unsupported API. classproperty docs doesn't need to contain it.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4b146e0c:

Fixed #30864 -- Doc'd classproperty decorator.