#30949: Use functools.cached_property instead of
django.utils.functional.cached_property.
-------------------------------------+-------------------------------------
Reporter: Thomas Grainger | Owner: David
Type: | Smith
Cleanup/optimization | Status: assigned
Component: Utilities | Version: dev
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Someday/Maybe
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Filip Sedlák):
One thing to note is that the `@functools.cached_property` supports a use
case that Django's doesn't. If you define a subclass with plain
`@property`, the caching doesn't happen because the `@property` is a "data
descriptor" so it gets precedence before the cached value. It's still not
clear to me why `super().count` also avoids the cached value in
`self.__dict__` but it clearly does.
{{{#!python
In [8]: class A:
...: @django.utils.functional.cached_property
...: def count(self):
...: print('Computing... A.count')
...: return 15
...:
...:
...: class B(A):
...: @property
...: def count(self):
...: print('B.count')
...: return super().count
...:
...:
In [9]: b = B()
In [10]: b.count
B.count
Computing... A.count <===== expected
Out[10]: 15
In [11]: b.count
B.count
Computing... A.count <===== should not happen
Out[11]: 15
}}}
The difference with `functools` is that `functools.cached_property` still
explicitly looks into `instance.__dict__` if it's called.
It's a corner case but I expected `super().count` to be cached. It hit us
when overriding `Paginator.count`. The lack of caching led to four count
queries. Small impact, easy workaround, but surprising behaviour.
--
Ticket URL: <https://code.djangoproject.com/ticket/30949#comment:13>
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