New submission from Laurie Opperman <laurie_opper...@hotmail.com>:
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
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue38553>
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