From 26be90d67de7bf5855a1312113288bcb1ee12051 Mon Sep 17 00:00:00 2001
From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Tue, 15 Apr 2025 13:38:03 +0300
Subject: [PATCH] gh-130664: support '_' (just as ',') in Decimal's formatting
 (GH-132155) (cherry picked from commit
 e10fe81cc6ae0979938eb3925139d56a74c620e3)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
---
 Lib/_pydecimal.py                                            | 2 +-
 Lib/test/test_decimal.py                                     | 5 +++++
 .../Library/2025-04-06-14-34-29.gh-issue-130664.JF2r-U.rst   | 2 ++
 3 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-04-06-14-34-29.gh-issue-130664.JF2r-U.rst

diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
index ff80180a79e97a..8d3a9f7ed45e25 100644
--- a/Lib/_pydecimal.py
+++ b/Lib/_pydecimal.py
@@ -6083,7 +6083,7 @@ def _convert_for_comparison(self, other, equality_op=False):
 (?P<alt>\#)?
 (?P<zeropad>0)?
 (?P<minimumwidth>(?!0)\d+)?
-(?P<thousands_sep>,)?
+(?P<thousands_sep>[,_])?
 (?:\.(?P<precision>0|(?!0)\d+))?
 (?P<type>[eEfFgGn%])?
 \Z
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 46c50115aef117..23107c603f8d41 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -1059,6 +1059,11 @@ def test_formatting(self):
             (',%', '123.456789', '12,345.6789%'),
             (',e', '123456', '1.23456e+5'),
             (',E', '123456', '1.23456E+5'),
+            # ... with '_' instead
+            ('_', '1234567', '1_234_567'),
+            ('07_', '1234.56', '1_234.56'),
+            ('_', '1.23456789', '1.23456789'),
+            ('_%', '123.456789', '12_345.6789%'),
 
             # negative zero: default behavior
             ('.1f', '-0', '-0.0'),
diff --git a/Misc/NEWS.d/next/Library/2025-04-06-14-34-29.gh-issue-130664.JF2r-U.rst b/Misc/NEWS.d/next/Library/2025-04-06-14-34-29.gh-issue-130664.JF2r-U.rst
new file mode 100644
index 00000000000000..294a7e031b2806
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-04-06-14-34-29.gh-issue-130664.JF2r-U.rst
@@ -0,0 +1,2 @@
+Support the ``'_'`` digit separator in formatting of the integral part of
+:class:`~decimal.Decimal`'s.  Patch by Sergey B Kirpichev.