[issue38553] Document functools.cached_property supports value updating and clearing

[Laurie Opperman]

Laurie Opperman  added the comment:

@hongweipeng What can be solved? Do you mean `functools.cached_property` should 
inherit `property`? First, this would break existing code dependant on the 
functionality of `functools.cached_property` (admittedly, Python 3.8 was 
release less than two weeks ago). Second, that seems like arbitrary 
restriction: I'd prefer just to update the documentation

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[issue38553] Document functools.cached_property supports value updating and clearing

[hongweipeng]

hongweipeng  added the comment:

It can be solved by extending propery. `class cached_property(property):` . How 
about this idea?

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nosy: +hongweipeng

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[issue38553] Document functools.cached_property supports value updating and clearing

[Laurie Opperman]

New submission from Laurie Opperman :

As I discovered during working on bpo-38545, `functools.cached_property` 
natively supports cached value setting/updating and clearing (via attribute 
assignment and deletion, respectively) through mechanisms defined in the 
language for non-data descriptors:

class A:
j = 0

@functools.cached_property
def b(self):
self.j += 1
return self.j

a = A()
print(a.b)  # 1
print(a.b)  # 1
del a.b
print(a.b)  # 2
print(a.b)  # 2
a.b = 42
print(a.b)  # 42
print(a.b)  # 42
del a.b
print(a.b)  # 3
print(a.b)  # 3

I propose that this functionality be documented, for two reasons:
1. To notify developers that this functionality is available, and how to access 
it
2. To notify developers that this functionality needs to be explicitly 
disabled. This is important as the built-in `property` is the reverse: property 
setting and deletion needs to be explicitly implemented

Disabling the value can be achieved by subclassing `functools.cached_property` 
and overriding/implementing `__set__` and `__delete__` to raise:

class immutable_cached_property(functools.cached_property):
def __set__(self, instance, value):
raise AttributeError("can't set attribute")

def __delete__(self, instance):
raise AttributeError("can't delete attribute")

Further reading:
- Defining descriptors: 
https://docs.python.org/3/reference/datamodel.html#implementing-descriptors
- Descriptor how-to: https://docs.python.org/3/howto/descriptor.html
- Class construction: 
https://docs.python.org/3/reference/datamodel.html#creating-the-class-object

--
assignee: docs@python
components: Documentation, Library (Lib)
messages: 355110
nosy: Epic_Wink, docs@python
priority: normal
severity: normal
status: open
title: Document functools.cached_property supports value updating and clearing
type: enhancement
versions: Python 3.8

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