Please consider mentioning property without setter when an attribute can't be set

[NeilGirdhar]

BPO	46730
Nosy	@rhettinger, @tiran, @NeilGirdhar, @tirkarthi, @sweeneyde, @Alex-Blade
PRs	bpo-46730: Add more info to @property AttributeError messages #31311 bpo-46730: Fix refleak and tighten NULL checks #31389 bpo-46730: Use raw string to avoid deprecation warning #31427

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-02-16.07:16:46.216>
created_at = <Date 2022-02-12.11:15:48.674>
labels = ['interpreter-core', '3.11']
title = "Please consider mentioning property without setter when an attribute can't be set"
updated_at = <Date 2022-02-19.05:36:47.897>
user = 'https://github.com/NeilGirdhar'

bugs.python.org fields:

activity = <Date 2022-02-19.05:36:47.897>
actor = 'Dennis Sweeney'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-16.07:16:46.216>
closer = 'Dennis Sweeney'
components = ['Interpreter Core']
creation = <Date 2022-02-12.11:15:48.674>
creator = 'NeilGirdhar'
dependencies = []
files = []
hgrepos = []
issue_num = 46730
keywords = ['patch']
message_count = 9.0
messages = ['413122', '413152', '413153', '413154', '413161', '413318', '413319', '413446', '413532']
nosy_count = 6.0
nosy_names = ['rhettinger', 'christian.heimes', 'NeilGirdhar', 'xtreak', 'Dennis Sweeney', 'Alex-Blade']
pr_nums = ['31311', '31389', '31427']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46730'
versions = ['Python 3.11']

[NeilGirdhar]

class C:
    @property
    def f(self) -> int:
        return 2

class D(C):
    pass

D().f = 2

Gives:

Traceback (most recent call last):
  File "/home/neil/src/cmm/a.py", line 10, in <module>
    D().f = 2
AttributeError: can't set attribute 'f'

This can be a pain to debug when the property is buried in a base class. Would it make sense to mention the reason why the attribute can't be set, namely that it's on a property without a setter?

[Alex-Blade]

Indeed, the error message does not help to identify the problem. Moreover, it collides with similar errors in namedtuple and DynamicClassAttribute which may lead to even more confusion.

I made a draft patch that could help with it (Alex-Blade@06df3a7), but I have a concern related to backwards compatibility (that's why no PR). I don't really understand if according to PEP-387 a change in an exception message should be considered compatibility breaking?

[NeilGirdhar]

Thank you, this would have saved me a lot of time!

On Sat, Feb 12, 2022 at 8:37 PM Alexander <report@bugs.python.org> wrote:

Alexander <lakeevveekal@gmail.com> added the comment:

Indeed, the error message does not help to identify the problem. Moreover,
it collides with similar errors in namedtuple and DynamicClassAttribute
which may lead to even more confusion.

I made a draft patch that could help with it (
Alex-Blade@06df3a7),
but I have a concern related to backwards compatibility (that's why no PR).
I don't really understand if according to PEP-387 a change in an exception
message should be considered compatibility breaking?

----------
nosy: +Alex-Blade

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue46730\>

---

[sweeneyde]

Go ahead and open a PR -- it makes it easier to discuss particular changes.

Regarding backwards-compatibility, error messages improvements are fair game for Python 3.11, we just shouldn't backport them to earlier versions.

We can also consider including the type of the relevant object in the error messages.

[Alex-Blade]

Added the PR. (I have signed the CLA, just haven't got the response yet, doesn't affect the discussion I guess)

[sweeneyde]

New changeset 0cb765b by Alex-Blade in branch 'main':
bpo-46730: Add more info to @Property AttributeError messages (GH-31311)
0cb765b

[sweeneyde]

Thanks for the PR!

[sweeneyde]

New changeset 9e06d03 by Christian Heimes in branch 'main':
bpo-46730: Fix refleak and tighten NULL checks (GH-31389)
9e06d03

[tirkarthi]

The test introduces a deprecation warning. It might be fixed by using raw string like the msg_format used in other test.

./python -Wall -m py_compile Lib/test/test_property.py
Lib/test/test_property.py:345: DeprecationWarning: invalid escape sequence '\.'
msg_format = "^property of 'PropertyUnreachableAttributeNoName\.cls' object {}$"