Mailing List Archive: [issue41523] functools.cached_property does not satisfy the property check

[issue41523] functools.cached_property does not satisfy the property check

Aug 11, 2020, 10:23 AM

Post #1 of 10 (101 views)

New submission from Bernat Gabor <gaborjbernat@gmail.com>:

from functools import cached_property, lru_cache

class A:
@property
def a(self): return ''

@cached_property
def b(self): return ''

@property
@lru_cache
def c(self):
return ""

print(isinstance(A.a, property))
print(isinstance(A.b, property))
print(isinstance(A.c, property))

True
False
True

I feel like cached property should be of type property, not?

----------
messages: 375188
nosy: Bernat Gabor
priority: normal
severity: normal
status: open
title: functools.cached_property does not satisfy the property check
versions: Python 3.10, Python 3.8, Python 3.9

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 11, 2020, 10:52 AM

Post #2 of 10 (101 views)

Serhiy Storchaka <storchaka+cpython@gmail.com> added the comment:

No, cached_property is a completely different class.

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nosy: +serhiy.storchaka

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 11, 2020, 11:12 AM

Post #3 of 10 (101 views)

Bernat Gabor <gaborjbernat@gmail.com> added the comment:

I understand under the hood they're differenet, however I think cached_property wanted to be a shorter/better version of @property @lru_cache which does passes this check. Tools expecting properties to pass that check now need to extend their check... and similarly now is no longer enough to use isinstance property to check if something is a property.

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 11, 2020, 11:46 AM

Post #4 of 10 (101 views)

Serhiy Storchaka <storchaka+cpython@gmail.com> added the comment:

There are not much things common to property and cached_property. cached_property does not have attributes fget, fset, fdel. They are both descriptors, but cached_property is not a data descriptor.

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 11, 2020, 12:18 PM

Post #5 of 10 (101 views)

Bernát Gábor <jokerjokerer@gmail.com> added the comment:

I think they're implemented differently and have slightly different semantics, but they're equal from a user point of view most of the time ????

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nosy: +Bernát Gábor

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 11, 2020, 12:45 PM

Post #6 of 10 (101 views)

William Pickard <lollol222gg@gmail.com> added the comment:

In the lru_cache example, I think property is using the result of 'lru_cache(c)', which in turns returns a property instance, not a subtype instance.

----------
nosy: +WildCard65

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 11, 2020, 12:56 PM

Post #7 of 10 (101 views)

Pablo Galindo Salgado <pablogsal@gmail.com> added the comment:

> They are both descriptors, but cached_property is not a data descriptor.

I think this is one of the key reasons this should not return "True" for isinstance(A.b, property). Also, cached_property does not allow you to implement a setter or a deleter.

The docs say "Similar to property()" but not "the same as property()"

----------
nosy: +pablogsal

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 17, 2020, 8:08 PM

Post #8 of 10 (91 views)

Raymond Hettinger <raymond.hettinger@gmail.com> added the comment:

I recommend closing this.

As Serhiy pointed out, they are profoundly different. Currently, a user can rely on the isinstance() check to differentiate them. So, changing the behavior would be a regression.

AFAICT, a user would not be able to deduce anything useful from the isinstance() check returning True in all three cases. Essentially, all they have in common is the concept of caching; otherwise, the mechanisms and implications are entirely unrelated.

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nosy: +rhettinger

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 17, 2020, 8:25 PM

Post #9 of 10 (91 views)

William Pickard <lollol222gg@gmail.com> added the comment:

Another thing to note Raymond, as I stated before, example C is, from an external context, is a plain property object who's "fget" attribute is an instance of whatever "lru_cache" returns.

isinstance won't be able to differentiate between example "a" and "c" without checking "fget"

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[issue41523] functools.cached_property does not satisfy the property check [ In reply to ]

Aug 17, 2020, 8:34 PM

Post #10 of 10 (91 views)

Change by Serhiy Storchaka <storchaka+cpython@gmail.com>:

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resolution: -> not a bug
stage: -> resolved
status: open -> closed

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