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

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

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

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 5ada827328188dfa097df04aff4b43a5ffa54c78..b2ea9c1781b870d2b5b575d90ad538ef0d28bcd9 100644 (file)
--- 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