tuple.index vs list.index different value errors

[xSpecialFoodx]

Bug report

Bug description:

().index(0)  # outputs "tuple.index(x): x not in tuple"
[].index(0)  # outputs "0 is not in list"

i think it'd be useful to normalize them to have the same value error, in tuple you can see that the item's value or name isn't even mentioned, unlike in list that the item's value is mentioned, although the name is not.

CPython versions tested on:

3.11

Operating systems tested on:

Windows

Linked PRs

• gh-121288: improve error messages for tuple.index, list.index and list.remove #121308
• gh-121288: Make error message for index() methods consistent #121395

[Eclips4]

Yes, the output from list.index is more obvious, in my opinion.
I think we should treat this issue as a type-feature rather than a type-bug, since there is no bug, it's just a place where we can improve the behavior.

[kumaraditya303]

+1 SGTM

[picnixz]

I'll take that one if possible

[picnixz]

By the way, for list.remove:

PyErr_SetString(PyExc_ValueError, "list.remove(x): x not in list");

So, should I also change this one?

[Eclips4]

Yes, let's also improve that one.

[picnixz]

After digging up a bit, I've concluded that this proposal has more or less been rejected previously (see #57558 (comment)). So I'm not sure I want to change it now.

[nineteendo]

Thanks for taking the time to look through previous issues. This tends to be forgotten.

[serhiy-storchaka]

The same issue is reported regularly:

• ValueError exception of tuple.index(x) gives imprecise error message #49745
• Non-informative error message in index() and remove() functions #57558
• Improve exception messages for remove and index methods #74039
• tuple.index error message improvement #74662
• tuple.index() could return a more explicit error message #77741
• Different ValueError for the same operation in List and Tuple #80623
• ValueError using index on tuple is not showing the tuple value #83680

I think that we should close it from other way: remove the repr of the value from error message for list.index().

[serhiy-storchaka]

On other hand, error message like "tuple.index(x): x not in tuple" is not especially good:

• it repeats the type name twice
• it uses "x"

Something like "value not found in ..." may look better. But I am not sure that it is worth to change error messages in all index() methods.

BTW, for str, bytes and bytearray the error messages are different: "substring not found" and "subsection not found".

[nineteendo]

How about "value not found"? That would also be consistent with str.index(), bytes.index(), and bytearray.index().

[vstinner]

Latest attempt to include repr(obj) in list error message went bad:

• Python 3.13: list.remove(x) performance degradation with missing x and expensive repr(x) #116792
• Revert "gh-96844: Improve error message of list.remove (gh-106455)" #116956
• Show value in error of list.remove #96844

[nineteendo]

Note that the issue was:

tuple.index vs list.index different value error

That's now fixed, so this should be closed as "completed".

[vstinner]

I didn't see that a PR was merged.