https://docs.python.org/3/library/string.html#formatstrings has the following -

An expression of the form '.name' selects the named attribute using [getattr()], while an expression of the form '[index]' does an index lookup using __getitem__().

An expression of the form [1] correctly returns the element in position 1. An expression of the form [-1] fails with TypeError: list indices must be integers or slices, not str.

Discussion on python-list (https://mail.python.org/pipermail/python-list/2022-July/906930.html) led to unearthing this quote from PEP3101 - Advanced String Formatting (https://peps.python.org/pep-3101/#simple-and-compound-field-names) -

The rules for parsing an item key are very simple. If it starts with a digit, then it is treated as a number, otherwise it is used as a string.

It was felt that the documentation would be improved if this rule was made explicit.

Linked PRs

• GH-95088: Clarified rules for parsing an item key for format strings. #103779
• [3.11] GH-95088: Clarify rules for parsing an item key for format strings (GH-103779) #105418
• [3.12] GH-95088: Clarify rules for parsing an item key for format strings (GH-103779) #105419