GH-95088: Clarified rules for parsing an item key for format strings.

[achhina]

Fixes GH-95088.

[JelleZijlstra]

Hold on, this isn't actually true:

>>> "{[0a]}".format([1])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers or slices, not str
>>> "{[0a]}".format({"0a": "x"})
'x'

This is implemented in

cpython/Objects/stringlib/unicode_format.h

Line 110 in fbf3596

static Py_ssize_t

. The actual implementation is equivalent to: if key.isdecimal(), treat it as an int, else treat it as a string.

That's different from what the PEP said, but it's probably a better idea, so I'd prefer to document this behavior.

[achhina]

The actual implementation is equivalent to: if key.isdecimal(), treat it as an int, else treat it as a string.

Ah, interesting! Okay updated it to be:

"A key is treated as a number if str(key).isdecimal() would evaluate to true."

[miss-islington]

Thanks @achhina for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington]

Sorry @achhina and @JelleZijlstra, I had trouble checking out the 3.12 backport branch.
Please retry by removing and re-adding the "needs backport to 3.12" label.
Alternatively, you can backport using cherry_picker on the command line.
cherry_picker 3e7316d7e8969febb56fbc7416d483b073bd1702 3.12

[bedevere-bot]

GH-105418 is a backport of this pull request to the 3.11 branch.

[miss-islington]

Thanks @achhina for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-105419 is a backport of this pull request to the 3.12 branch.