[issue27823] Change bare AttributeError messages to be more informative

[Anilyka Barry]

Anilyka Barry  added the comment:

I'd forgotten that I did that. Looking back on it, this is indeed not a good 
change. Since there hasn't been any traction on in for 5 years, I think it's 
safe to say it's not something people want.

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

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[issue27823] Change bare AttributeError messages to be more informative

[Irit Katriel]

Irit Katriel  added the comment:

-1

As Anilyka wrote: "it (AttributeError) doesn't even mean the attribute doesn't 
exist, as it may".

So it's not safe to fill in a missing message based on the assumption that we 
know what it should be.

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nosy: +iritkatriel

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[issue27823] Change bare AttributeError messages to be more informative

[Emanuel Barry]

Emanuel Barry added the comment:

Thanks for your comments Xiang. Yes, it's not equal to #27794, but it's one of 
the multiple ways to fix it, so I made a new issue about it.

The rationale between a bare `raise AttributeError` being changed is the idea 
that it carries no information, other than the fact there was an attribute 
error (it doesn't even mean the attribute doesn't exist, as it may). Also note 
that this only affects bare AttributeErrors inside of the magic methods 
dedicated to attribute lookup (and the descriptor machinery). In a regular 
function (or even just doing `foo.__getattribute__(bar)`), `raise 
AttributeError` does nothing special.

The argument "If you do want a good message, why not passing the message when 
raising?" is one I hear a lot when there's a suggestion to add a more 
convenient way to do something that can already be done. I've seen a lot of 
code (including code in the core Python distribution, both in Python and C) 
which lazily does `raise AttributeError(name)`. It's only half the information, 
and these uninformative error messages are left completely untouched by my 
patch. The patch will help reduce boilerplate in some code. I'm personally one 
of those people who go ahead and actually use the built-in error message in my 
own code, but if I can reduce the boilerplate needed while maintaining the same 
level of information and usefulness for debugging, I'll be happy.

On the other hand, the argument that Python hides away some magic is a valid 
concern. Python already has some magic in various places; the point is to 
determine whether this kind of magic is acceptable for Python and its users. I 
honestly don't know; I don't think the magic is too strong, but at the same 
time it's likely to surprise newcomers. We'll see what others think :)

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[issue27823] Change bare AttributeError messages to be more informative

[Xiang Zhang]

Xiang Zhang added the comment:

-1. I don't think this is equal to issue27794. I don't hope when I use `raise 
AttributeError`, Python hides some magic and sets a message I totally don't 
want. If you do want a good message, why not passing the message when raising?

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[issue27823] Change bare AttributeError messages to be more informative

[Xiang Zhang]

Changes by Xiang Zhang :


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[issue27823] Change bare AttributeError messages to be more informative

[Emanuel Barry]

New submission from Emanuel Barry:

Attached patch changes bare attribute errors to add a useful error message. As 
such, `raise AttributeError` and `raise AttributeError(None)` (but not `raise 
AttributeError('')` for example) inside any of the attribute lookup or 
descriptor methods (__getattribute__, __getattr__, __setattr__, __delattr__, 
__get__, __set__, and __delete__). This is a followup to #27794.

As such, the following is now much more useful and less boilerplate:

class A:
_dynamic_names = {}
def __getattr__(self, name):
if name in self._dynamic_names:
return self._dynamic_names[name]
raise AttributeError

Then, doing the following:

A().spam

Will result in a `AttributeError("'A' object has no attribute 'spam'")`, while 
keeping the original traceback. The patch is pretty much complete, but I have a 
few concerns:

- Performance hit, especially on something that's used virtually everywhere and 
everytime (attribute access); although PEP 487's __set_name__ lookup already 
hurts performance slightly. The few micro-benchmarks I ran weren't very 
significant, but there is a small drop in performance nonetheless.
- Four new PyErr_* API functions, one which isn't used in the current patch, 
but since it's 3 lines I decided to leave it in anyway.
- I don't carry over the cause or context of the raised exception to the new 
one. Keeping the traceback around is pretty trivial, since it has its own slot 
in the thread state struct, but since not all exceptions are actual exceptions 
at the C level (and the new exceptions I raise aren't either, at the C level), 
I don't know how to carry over context or cause.

Oh, and exceptions set at the C level with e.g. `PyErr_SetNone` are also 
properly handled that way; this would fix #27794 as a side-effect, but a few 
lines will need to be changed (in Objects/descrobject.c) for this to work. Such 
changes are not included with the patch.

Thoughts, concerns, opinions, ideas? Maybe something I didn't think of?

--
components: Interpreter Core
files: change_attr_error_messages_1.patch
keywords: patch
messages: 273300
nosy: ebarry, r.david.murray, xiang.zhang, ztane
priority: normal
severity: normal
stage: patch review
status: open
title: Change bare AttributeError messages to be more informative
type: enhancement
versions: Python 3.6
Added file: http://bugs.python.org/file44180/change_attr_error_messages_1.patch

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