[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Tal Einat  added the comment:

Thanks for the suggestion hongweipeng!

Thanks for the PR Florian, and congratulations on your first contribution to 
the project! May it be the first of many.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

miss-islington  added the comment:


New changeset c0db88f6abbace79644b2aca2290bf41b1a37174 by Miss Islington (bot) 
in branch '3.8':
bpo-38524: clarify example a bit and improve formatting (GH-17406)
https://github.com/python/cpython/commit/c0db88f6abbace79644b2aca2290bf41b1a37174


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

miss-islington  added the comment:


New changeset 7e9bbbe51e74e5928e6a6c3e70434d824970ef58 by Miss Islington (bot) 
in branch '3.7':
bpo-38524: clarify example a bit and improve formatting (GH-17406)
https://github.com/python/cpython/commit/7e9bbbe51e74e5928e6a6c3e70434d824970ef58


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

Change by miss-islington :


--
pull_requests: +16892
pull_request: https://github.com/python/cpython/pull/17412

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

Change by miss-islington :


--
pull_requests: +16891
pull_request: https://github.com/python/cpython/pull/17411

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Tal Einat  added the comment:


New changeset 02519f75d15b063914a11351da30178ca4ceb54b by Tal Einat in branch 
'master':
bpo-38524: clarify example a bit and improve formatting (GH-17406)
https://github.com/python/cpython/commit/02519f75d15b063914a11351da30178ca4ceb54b


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Change by Tal Einat :


--
pull_requests: +16886
pull_request: https://github.com/python/cpython/pull/17406

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

miss-islington  added the comment:


New changeset 0f9c9d53283420a570850aa92869d032b40d4fba by Miss Islington (bot) 
in branch '3.8':
bpo-38524: document implicit and explicit calling of descriptors' __set_name__ 
(GH-17364)
https://github.com/python/cpython/commit/0f9c9d53283420a570850aa92869d032b40d4fba


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

miss-islington  added the comment:


New changeset cd27d22ac90a869dc740004597246f24246348a6 by Miss Islington (bot) 
in branch '3.7':
bpo-38524: document implicit and explicit calling of descriptors' __set_name__ 
(GH-17364)
https://github.com/python/cpython/commit/cd27d22ac90a869dc740004597246f24246348a6


--
nosy: +miss-islington

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

Change by miss-islington :


--
pull_requests: +16882
pull_request: https://github.com/python/cpython/pull/17402

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[miss-islington]

Change by miss-islington :


--
pull_requests: +16881
pull_request: https://github.com/python/cpython/pull/17401

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Tal Einat  added the comment:


New changeset 1bddf890e595a865414645c6041733043c4081f8 by Tal Einat (Florian 
Dahlitz) in branch 'master':
bpo-38524: document implicit and explicit calling of descriptors' __set_name__ 
(GH-17364)
https://github.com/python/cpython/commit/1bddf890e595a865414645c6041733043c4081f8


--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Change by Tal Einat :


--
versions: +Python 3.7, Python 3.9

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Florian Dahlitz]

Change by Florian Dahlitz :


--
keywords: +patch
pull_requests: +16849
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17364

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Tal Einat  added the comment:

Hi Florian! We'd be happy to have you do this, please go ahead.

When you have a PR ready, you're welcome to request my review.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Florian Dahlitz]

Florian Dahlitz  added the comment:

It would be an honor for me to work on this issue (updating the docs) as my 
first CPython contribution.

--
nosy: +DahlitzFlorian

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Tal Einat]

Tal Einat  added the comment:

I agree with Nick: While possible, this would be unnecessarily confusing.

As Serhiy wrote, the proper way to address the situation seems to be improving 
the documentation. A PR would be welcome!

--
keywords: +easy
nosy: +taleinat

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Sergey Fedoseev]

Change by Sergey Fedoseev :


--
nosy: +sir-sigurd

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Nick Coghlan]

Nick Coghlan  added the comment:

After writing my previous comment, I double-checked the code, and 
cached_propery is actually one of the cases where a simple "self.attrname = 
'age3'" *will* do the right thing, as cached_property never checks the class 
information.

That won't work in the general case though, and I think it makes more sense for 
the cached_property documentation to provide advice that will generalise to 
arbitrary descriptors (i.e. call `descr.__set_name__(target, "attr")`)

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Nick Coghlan]

Nick Coghlan  added the comment:

Regarding the "attrname" property idea: unfortunately, that won't work, as 
`__set_name__` doesn't just provide the attribute name, it also provides a 
reference to the class itself.

cached_property needs both pieces of information, not just the attribute name.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Nick Coghlan]

Nick Coghlan  added the comment:

Another interesting question this raises is whether type.__setattr__ should be 
checking for values that have `__set_name__` methods defined and calling those 
methods automatically.

Unfortunately, I think the answer to that is "If we'd thought of that when 
writing PEP 487, then sure, but it's too late now, as too many folks will be 
calling it explicitly, and implicitly calling it a second time will cause 
other, potentially harder to find problems".

So +1 for updating the cached_property docs specifically to mention this problem

However, I'm also wondering if there's somewhere else we should be mentioning 
it as a general problem.

Perhaps in the docs for __set_name__ itself, noting it as something that 
authors of descriptors *using* __set_name__ should mention in their docs, with 
suggested wording?

Something like:

===
`__set_name__`` is only called implicitly as part of the ``type`` constructor, 
so it will need to be called explicitly with the appropriate parameters when a 
descriptor is added to a class after initial creation::

descr = custom_descriptor()
cls.attr = descr
descr.__set_name__(cls, 'attr')
===

(The normal sequence is for the descriptor to already be part of the class 
namespace before __set_name__ gets called)

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

See issue38517.

For now we need to better document the use of __set_name__ with cached_property.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[hongweipeng]

hongweipeng  added the comment:

Is it possible to add an attrname parameter? It is more convenient to use.

class cached_property:
def __init__(self, func, attrname=None):
self.attrname = attrname
...

cls.age3 = cached_property(age, 'age3')

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

You have to call __set_name__() if you create a cached_property dynamically 
outside of the class namespace.

p = cached_property(age)
p.__set_name__(cls, 'age3')
cls.age3 = p

(And setattr() is not needed here.)

--
nosy: +serhiy.storchaka

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
nosy: +carljm, ncoghlan

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[Steven D'Aprano]

Steven D'Aprano  added the comment:

The documentation doesn't mention ``__set_name__``, but it does say that 
cached_property is useful for properties which are "effectively immutable".

The ``__set_name__`` error message is pretty cryptic, that seems to have 
something to do with this PEP:

https://www.python.org/dev/peps/pep-0487/

but why it is relevant here, I don't know. I would expect that cached_property 
should apply only to read-only properties.

So I think that at the very least, this needs better documentation and a better 
error message.

--
assignee:  -> docs@python
components: +Documentation
nosy: +docs@python, steven.daprano

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue38524] functools.cached_property is not supported for setattr

[hongweipeng]

New submission from hongweipeng :

```
from functools import cached_property
def age(self):
return 10
class A:
def __init__(self):
setattr(self.__class__, 'age', property(age))
setattr(self.__class__, 'age3', cached_property(age))

age2 = cached_property(age)

a = A()
print(a.age)# 10
print(a.age2)   # 10
print(a.age3)   # TypeError: Cannot use cached_property instance without 
calling __set_name__
```
Is it expected?

--
messages: 354929
nosy: hongweipeng
priority: normal
severity: normal
status: open
title: functools.cached_property is not supported for setattr
versions: Python 3.8

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com