str.format() raises misleading error when using negative indices

[makukha]

Bug report

Bug description:

This is a duplicate of #95846. The problem appeared on Stackoverflow, but I didn't manage to reopen the old issue.

>>> x = [1,2,3]
>>> '{x[-1]}'.format(x=x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers or slices, not str
>>> x = (1,2,3)
>>> '{x[-1]}'.format(x=x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: tuple indices must be integers or slices, not str

Expected results:

• Correct error message: TypeError: ___ indices must be non-negative integers or slices
• Documentation updated to clearly state that list indices can't be negative.

As long as this issue is not critical, I would be glad to provide PR.

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Linked PRs

• gh-124294: str.format() raises misleading error when using negative indices #124372

[rruuaanng]

This issue was raised in #95846.

Proposed by JelleZijlstra, this modification may compromise backward compatibility, and I suggest that perhaps this issue can be ignored. perhaps you can modify this warning message.

[makukha]

@rruuaanng

1. Can error message be updated like "... indices must be non-negative integers or slices"?
2. Should docs be updated to clearly state that only non-negative integers are allowed?

[rruuaanng]

@rruuaanng

1. Can error message be updated like "... indices must be non-negative integers or slices"?

2. Should docs be updated to clearly state that only non-negative integers are allowed?

I think we could add an extra tip, like suggesting the use of f'...' syntax, or mention that this syntax doesn't allow negative indexing. if we specify that negative indexing isn't allowed, it might confuse other users when they're using the equivalent f'...' syntax.

Maybe this issue should be left for the core developers to answer.

[ericvsmith]

It's not so easy to modify the exception message, because where it's raised there's no knowledge of what the caller is expecting. And duplicating the logic at the caller site just to change the error message would be a maintenance burden. But if you come up with a patch I'd look at it.

[ericvsmith]

Per the discussion on the PR, I'm closing this.