Improve string's __getitem__ error message

[miguendes]

BPO	44110
Nosy	@serhiy-storchaka, @miguendes
PRs	bpo-44110: Improve string's __getitem__ error message #26042

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-06-27.12:12:22.594>
created_at = <Date 2021-05-11.20:50:25.367>
labels = ['interpreter-core', 'type-feature', '3.11']
title = "Improve string's __getitem__ error message"
updated_at = <Date 2021-06-27.12:14:59.005>
user = 'https://github.com/miguendes'

bugs.python.org fields:

activity = <Date 2021-06-27.12:14:59.005>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-27.12:12:22.594>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2021-05-11.20:50:25.367>
creator = 'miguendes'
dependencies = []
files = []
hgrepos = []
issue_num = 44110
keywords = ['patch']
message_count = 3.0
messages = ['393473', '396575', '396577']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'miguendes']
pr_nums = ['26042']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44110'
versions = ['Python 3.11']

[miguendes]

I noticed that getitem message, although helpful, could be improved a bit further. This will also make it consistent with other error messages such as the ones raised by str.count, str.split, str.endswith and so many others.

Currently, the error message goes like this: "TypeError: string indices must be integers" but we could specify the type of the object passed as argument to __getitem__. So, for example:

>>> idx = '1'
>>> s = 'abcde'
>>> s[idx]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: string indices must be integers, not 'str'

This makes easier to debug and it is also consistent with other methods:

>>> "alala".count(8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str, not int

>>> "lala|helo".split(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str or None, not int

>>> 1 in "lala"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand, not int

>>> "lala|helo".split(object())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str or None, not object

[serhiy-storchaka]

New changeset ed10764 by Miguel Brito in branch 'main':
bpo-44110: Improve string's __getitem__ error message (GH-26042)
ed10764

[serhiy-storchaka]

Thank you for your contribution Miguel.

I decided to not backport this change to 3.10 because the benefit is too small in comparison with possibility to break someone's tests.