[issue18162] Add index attribute to IndexError

[Ezio Melotti]

Ezio Melotti added the comment:

See also #1182143.

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[issue18162] Add index attribute to IndexError

[Ezio Melotti]

Ezio Melotti added the comment:

And PEP 473.

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[issue18162] Add index attribute to IndexError

[Ofer Schwarz]

Ofer Schwarz added the comment:

I'm working on this now (patch hopefully coming soon), and I've hit an 
interesting issue:
PySequence methods take care of negative indexes by adding +len before calling 
the subclass method (s.t. -1 becomes len-1 etc.). This means that if the 
subclass raises an exception with the index, it might be the wrong index.

This case only happens when index  -len, so when I talked it out with Larry he 
suggested just passing the original index to the subclass in this case. So for 
index = 0 or index  -len the subclass will receive the actual index, and for 
-len = index  0 the subclass will get index+len.
That's a change in behavior, but I actually think it's a good one. The 
negative-index use case in PySequence is exactly intended for this [-len, 0) 
use case, and if some subclass wants to do something weird with negative 
indexes it'd probably want the actual argument anyway.

I'm writing my patch with this in it, but it might be worth discussing here if 
someone disagrees.

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[issue18162] Add index attribute to IndexError

[Ram Rachum]

Ram Rachum added the comment:

Looking forward to your patch Ofer! Thanks for working on this!

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[issue18162] Add index attribute to IndexError

[Ofer Schwarz]

Changes by Ofer Schwarz os.uran...@gmail.com:


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keywords: +patch
Added file: http://bugs.python.org/file39055/indexerror.patch

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[issue18162] Add index attribute to IndexError

[Ofer Schwarz]

Changes by Ofer Schwarz os.uran...@gmail.com:


Added file: http://bugs.python.org/file39056/indexerror.tests

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[issue18162] Add index attribute to IndexError

[Ofer Schwarz]

Changes by Ofer Schwarz os.uran...@gmail.com:


Added file: http://bugs.python.org/file39060/indexerror_all.patch

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[issue18162] Add index attribute to IndexError

[Ram Rachum]

Ram Rachum added the comment:

Since #21911 has been merged into this issue, I'd like to add: Please also 
include the length of the sequence in the exception message. It can help with 
debugging.

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[issue18162] Add index attribute to IndexError

[Ezio Melotti]

Ezio Melotti added the comment:

Knowing the len of the sequence would also be useful.
Brett, were you also planning to use these attributes in the error message 
(when they are available), or do you prefer to keep the two issues separate and 
reopen #21911?

 Is there a meta-issue for these changes?

FTR the other relevant issues are:
  #18156: Add an 'attr' attribute to AttributeError
  #18163: Add a 'key' attribute to KeyError
  #18165: Add 'unexpected_type' to TypeError
  #18166: 'value' attribute for ValueError

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[issue18162] Add index attribute to IndexError

[Brett Cannon]

Brett Cannon added the comment:

Part of the point of these various attributes I proposed was so that a good 
default message could be provided when only the new attributes are given. So 
I'm fine with that being part of this issue.

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[issue18162] Add index attribute to IndexError

[Yury Selivanov]

Changes by Yury Selivanov yselivanov...@gmail.com:


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versions: +Python 3.5 -Python 3.4

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[issue18162] Add index attribute to IndexError

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

See also issue1534607.

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[issue18162] Add index attribute to IndexError

[Brett Cannon]

Brett Cannon added the comment:

Do you mean sometimes there sometimes not because old code won't set it (yet) 
or because you don't think it will always be appropriate to set the attribute 
and thus people won't set it when available?

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[issue18162] Add index attribute to IndexError

[R. David Murray]

R. David Murray added the comment:

I am in favor of adding meaningful attributes to stdlib exceptions.  I've 
always considered the lack of such an API a wart in Python, though an 
understandable one (since exceptions started out as simple strings).  But yeah, 
while I hate to say it, this is probably mini-PEP material :(.

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[issue18162] Add index attribute to IndexError

[Raymond Hettinger]

Raymond Hettinger added the comment:

-1 on expanding the API for an attribute that is sometimes there and sometimes 
not.  This doesn't add any value, but it does add complication.

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[issue18162] Add index attribute to IndexError

[Ezio Melotti]

Ezio Melotti added the comment:

Is there a meta-issue for these changes?
I remember a similar discussion a couple of years ago, but I don't remember if 
it was on python-dev/ideas or on the bug tracker.
I agree that exceptions could be improved, but I would like to get the big 
picture of the changes you are planning to do instead of many individual 
issues.  A simple PEP might also be good, depending on how far you want to go :)

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[issue18162] Add index attribute to IndexError

[Brett Cannon]

Brett Cannon added the comment:

Nope, no meta-issue. I literally just realized one evening that the handful of 
exceptions that I filed bugs for could use an attribute for why the exception 
was raised.

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[issue18162] Add index attribute to IndexError

[Raymond Hettinger]

Raymond Hettinger added the comment:

I would also like to see a PEP or good python-dev discussion before embarking 
on all these API expansions.  There should be an evaluation of whether any 
existing code would benefit from it, whether the current args attribute is 
sufficient, whether the code bloat is worth it, etc.

Also, I'm unclear on whether you expect users to have to provide the index 
value in their own code (i.e. is raise IndexError to still be permitted or 
will all pure python code be required to use raise IndexError(i) and change 
existing code if they already use raise IndexError(Out of range)).

If you don't change requirements for pure python code, then how can be even 
rely on the new attribute being there (i.e.  is except IndexError as e:  
print(e.index) guaranteed to work?).

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[issue18162] Add index attribute to IndexError

[Brett Cannon]

Brett Cannon added the comment:

Obviously it can't be required that the index be provided as that would break 
way too much code. There are already exceptions in the stdlib that have 
optional attributes you can choose to (not) set.

As for relying upon it, it would be just like any other object that gets a new 
attribute: either you rely on it because you know the code you are using always 
provides it (e.g. using core/stdlib code directly) or you use some way to deal 
with cases where it was not set (EAFP or LBYL). Subclassing IndexError to 
provide a inheritance guarantee that an attribute exists seems like overkill 
(e.g. IndexError2 so you can do ``except IndexError2 as exc:``).

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[issue18162] Add index attribute to IndexError

[Chris Rebert]

Changes by Chris Rebert pyb...@rebertia.com:


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[issue18162] Add index attribute to IndexError

[Brett Cannon]

Brett Cannon added the comment:

Yes, I mean weakref.

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[issue18162] Add index attribute to IndexError

[Éric Araujo]

Éric Araujo added the comment:

regex?  do you mean weakref?

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[issue18162] Add index attribute to IndexError

[Brett Cannon]

New submission from Brett Cannon:

Give IndexError an index attribute to store the index it was raised for. Since 
it is typically an integer there is no reason to worry about GC and thus using 
a regex.

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components: Interpreter Core
messages: 190777
nosy: brett.cannon
priority: normal
severity: normal
stage: test needed
status: open
title: Add index attribute to IndexError
type: enhancement
versions: Python 3.4

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[issue18162] Add index attribute to IndexError

[Barry A. Warsaw]

Changes by Barry A. Warsaw ba...@python.org:


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