[Python-checkins] [3.11] GH-95088: Clarify rules for parsing an item key for format strings (GH-103779) (#105418)

[JelleZijlstra]

https://github.com/python/cpython/commit/5e4c46f02951b979854121adc4b628e57467ac95
commit: 5e4c46f02951b979854121adc4b628e57467ac95
branch: 3.11
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: JelleZijlstra <jelle.zijlstra at gmail.com>
date: 2023-06-06T20:56:03-07:00
summary:

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

GH-95088: Clarify rules for parsing an item key for format strings (GH-103779)
(cherry picked from commit 3e7316d7e8969febb56fbc7416d483b073bd1702)

Co-authored-by: achhina <amanschhina at gmail.com>
Co-authored-by: Łukasz Langa <lukasz at langa.pl>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra at gmail.com>

files:
M Doc/library/string.rst

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 5ada82732818..b2ea9c1781b8 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -227,7 +227,9 @@ See also the :ref:`formatspec` section.
 
 The *field_name* itself begins with an *arg_name* that is either a number or a
 keyword.  If it's a number, it refers to a positional argument, and if it's a keyword,
-it refers to a named keyword argument.  If the numerical arg_names in a format string
+it refers to a named keyword argument. An *arg_name* is treated as a number if
+a call to :meth:`str.isdecimal` on the string would return true.
+If the numerical arg_names in a format string
 are 0, 1, 2, ... in sequence, they can all be omitted (not just some)
 and the numbers 0, 1, 2, ... will be automatically inserted in that order.
 Because *arg_name* is not quote-delimited, it is not possible to specify arbitrary