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

[JelleZijlstra]

https://github.com/python/cpython/commit/3e7316d7e8969febb56fbc7416d483b073bd1702
commit: 3e7316d7e8969febb56fbc7416d483b073bd1702
branch: main
author: achhina <amanschhina at gmail.com>
committer: JelleZijlstra <jelle.zijlstra at gmail.com>
date: 2023-06-06T19:01:36-07:00
summary:

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

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 26b3f5000634..9b28f99536a3 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