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?

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 (https://github.com/Alex-Blade/cpython/commit/06df3a72dfe462c8fe4eac60dce0ef059b1738f8), 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?

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 (
> https://github.com/Alex-Blade/cpython/commit/06df3a72dfe462c8fe4eac60dce0ef059b1738f8),
> 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>
> _______________________________________
>

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.

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

New changeset 0cb765b2cec9b020224af016a83bf35c45b71932 by Alex-Blade in branch 'main':
bpo-46730: Add more info to @property AttributeError messages (GH-31311)
https://github.com/python/cpython/commit/0cb765b2cec9b020224af016a83bf35c45b71932

Thanks for the PR!

New changeset 9e06d03672547041239812efe4901c06da6cbd2f by Christian Heimes in branch 'main':
bpo-46730: Fix refleak and tighten NULL checks (GH-31389)
https://github.com/python/cpython/commit/9e06d03672547041239812efe4901c06da6cbd2f

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 {}$"