[issue44166] Make IndexError messages for list more informative

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Thank for the suggestion.  While we're going to a pass on this one, no doubt 
there are other places that language can improve its communication with the 
user.  Please continue to submit ideas and patches.

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[issue44166] Make IndexError messages for list more informative

[Raymond Hettinger]

Change by Raymond Hettinger :


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[issue44166] Make IndexError messages for list more informative

[Raymond Hettinger]

Change by Raymond Hettinger :


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

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[issue44166] Make IndexError messages for list more informative

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

I concur with Serhiy and Raymond and I would prefer to leave as is.

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[issue44166] Make IndexError messages for list more informative

[Vedran Čačić]

Vedran Čačić  added the comment:

> fix typically isn't replacing s[i] with s[i - 5]

... especially since that will still raise IndexError (in case when i==15 and 
len(s)==10). ;-P

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[issue44166] Make IndexError messages for list more informative

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Here's another way to think of it:  

The call s[i] raising an IndexError isn't a numerical error, it is a conceptual 
error.  Knowing that i==15 and len(s)==10 doesn't usually help resolve the 
problem.  The fix typically isn't replacing s[i] with s[i - 5].  Usually the 
issue is usually deeper than that.

Note, this is different than KeyError where misspellings are common and where 
the fix can be indicated by knowing the key:   d['missteak'] -> d['mistake'].

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[issue44166] Make IndexError messages for list more informative

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I also share Serhiy's concerns and prefer that it be left as-is.

Conceptually, adding more information in the error message would make it more 
useful for debugging, but in practice, it would rarely be helpful.  In teaching 
and coaching Python, I've learned that the intended, dominant effect of seeing 
"IndexError: list index out of range" is that a person understands that index 
is invalid because it is out of range.  That is a helpful dominant impression.  
IMO it would be distracting and rarely helpful to focus on the specific index 
and length.  The important point is that a nonsense lookup was being made.

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[issue44166] Make IndexError messages for list more informative

[Spencer Brown]

Spencer Brown  added the comment:

One potential solution would be to add two Py_ssize_t to IndexError, storing 
the index and length along with the existing exception value. Then __str__() 
can append that to the message if set, perhaps having len be negative to signal 
they're not passed. 

An issue though is that it might be backwards incompatible with classes trying 
to multiply inherit from other exceptions too. To deal with that it could put 
the values in the args tuple, though then it'd have to allocate the tuple and 
ints which might hurt perf.

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nosy: +Spencer Brown

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[issue44166] Make IndexError messages for list more informative

[Andre Roberge]

Change by Andre Roberge :


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nosy: +aroberge

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[issue44166] Make IndexError messages for list more informative

[Akash Mishra]

Akash Mishra  added the comment:

While browsing some traceback error reporting i land on this solution threads. 

Specially beginner face traceback error for 
- KeyError
- IndexError (As mentioned in current message thread) 
- ValueError
..

It is sometime self explanatory from error reports but may be it can help to 
design some error document in python to more generalize errors.

Found some of traceback error with some possible cause explanations. Sharing if 
this is useful.  

https://github.com/pablorgarcia/Learn-to-program-with-Python-guide/blob/master/Understanding%20errors/IndexError-list-index-out-of-range.py

https://www.quizcure.com/python/traceback-most-recent-call-last-in-python#Keyerror-dictionary

https://stackoverflow.com/questions/64428188/deep-learning-chatbot-specific-index-error-list-index-out-of-range

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nosy: +mishra.akash.myself

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[issue44166] Make IndexError messages for list more informative

[Rémi Lapeyre]

Change by Rémi Lapeyre :


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nosy: +remi.lapeyre

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[issue44166] Make IndexError messages for list more informative

[Miguel Brito]

Miguel Brito  added the comment:

Thanks for your comments, folks! I really appreciate it.

I left a comment about this issue in the PR thread: 
https://github.com/python/cpython/pull/26207#issuecomment-843531990

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[issue44166] Make IndexError messages for list more informative

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

I share the same concern as Serhiy, see my comment in the PR:

https://github.com/python/cpython/pull/26207#pullrequestreview-661799899

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[issue44166] Make IndexError messages for list more informative

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The problem is that such exceptions are often used with the EAFP style. For 
example:

while stop is None or i < stop:
try:
v = self[i]
if v is value or v == value:
return i
except IndexError:
break
i += 1

You caught IndexError and do something, but do not use the error message. 
Formatting error message takes a time. I am sure that error messages of a wast 
majority of IndexError exceptions are not used, just the type of the raised 
exception matters.

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nosy: +serhiy.storchaka

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[issue44166] Make IndexError messages for list more informative

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +pablogsal

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[issue44166] Make IndexError messages for list more informative

[Miguel Brito]

Change by Miguel Brito :


--
keywords: +patch
pull_requests: +24824
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/26207

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[issue44166] Make IndexError messages for list more informative

[Miguel Brito]

New submission from Miguel Brito :

I've noticed that in people will often ask for help on forums or stackoverflow 
to better understand the causes of
IndexErrors [1][2][3].
> The error message line for an IndexError doesn’t give you great 
> information.[1]

Currently, when we access a out of bounds position we get something along the 
lines:
Traceback (most recent call last):

>>> a = [0, 10, 20, 30, 40]
>>> a[5]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range
>>> a.pop(6)
Traceback (most recent call last):
  File "", line 1, in 
IndexError: pop index out of range
>>> a[6] = 7
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list assignment index out of range
>>> a = []
>>> a[2]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range


These error messages are too broad and doesn't inform the current length of the 
list, nor the index that was fed to it.

To improve the debugging experience in both interactive and non-interactive 
code,
I propose to offer a richer and more informative error messages. I think this 
can help users, especially newer and less
experienced folks, debug their code.


>>> a = [0, 10, 20, 30, 40]
>>> a[5]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range, the len is 5 so index must be in -5..4, 
got 5
>>> a[-6]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range, the len is 5 so index must be in -5..4, 
got -6
>>> a.pop(6)
Traceback (most recent call last):
  File "", line 1, in 
IndexError: pop index out of range, the len is 5 so index must be in -5..4, got 
6
>>> a[6] = 7
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list assignment index out of range, the len is 5 so index must be 
in -5..4, got 6
>>> a = []
>>> a[2] = 0
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list assignment index out of range, got index 2 but the list is 
empty


These proposed messages are inspired by other languages, with the difference 
that in Python we can have negative index.
So informing the allowed ranges is a plus.

These improvements are not restricted to list, so it can be applied to strings 
and tuples as well, or any other indexable
object.

I have a branch ready, looking forward to hearing from you!

[1] 
https://stackoverflow.com/questions/1098643/indexerror-list-index-out-of-range-and-python
[2] 
https://stackoverflow.com/questions/16005707/index-error-list-index-out-of-range-python
[3] https://realpython.com/python-traceback/#indexerror

--
components: Interpreter Core
messages: 393858
nosy: miguendes
priority: normal
severity: normal
status: open
title: Make IndexError messages for list more informative
type: enhancement
versions: Python 3.11

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