On Thu, Feb 10, 2022 at 02:27:42PM -0800, Neil Girdhar wrote:

> 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?

I have no objection to changing the error message, I'm sure it's a small
enough change that you should just open a ticket on b.p.o. for it. But I
don't expect that it will be particularly useful either.

If you can't set an attribute on an object, aren't there three obvious
causes to check?

- the object has no __dict__, and so has no attributes at all;
  e.g. trying to set an attribute on a float;

- the object has slots, but 'f' is not one of them;

- or 'f' is a property with no setter (or a setter that raises
  AttributeError).

Have I missed any common cases?

The error messages are different in each case, but even if they were the
same, there are three obvious causes to check, and property is one of
them.

I suppose that there are exotic failures that could happen in
__getattribute__ or weird descriptors, or metaclass magic, etc, and
*those* might be hard to debug, but it doesn't seem likely to me that a
read-only property will be mysterious.

Especially if you have the object available in the interactive
interpreter, so you can inspect it with the various tools available:

* dir(obj)
* help(obj)
* type(obj).f  # Will display <property object at 0x...>

etc. Its hard to keep the existence of a property secret in Python.

So once you know that f is a property, it might be hard to work out
which of the fifty-seven superclasses and mixins it came from *wink* but
that's neither here nor there :-)

Maybe reporting "can't set property 'f'" is good enough.


--
Steve
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