From 9cac4515afa64379ed20ea683ced40d9c028fd4d Mon Sep 17 00:00:00 2001
From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Mon, 7 Apr 2025 09:54:06 +0300
Subject: [PATCH] gh-87790: support thousands separators for formatting
 fractional part of Decimal's

---
 Lib/_pydecimal.py                                  | 14 +++++++++++++-
 Lib/test/test_decimal.py                           | 12 ++++++++++++
 .../2025-04-07-09-53-54.gh-issue-87790.6nj3zQ.rst  |  2 ++
 3 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-04-07-09-53-54.gh-issue-87790.6nj3zQ.rst

diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
index ec036199331396..4b41826146e7ca 100644
--- a/Lib/_pydecimal.py
+++ b/Lib/_pydecimal.py
@@ -6099,7 +6099,11 @@ def _convert_for_comparison(self, other, equality_op=False):
 (?P<zeropad>0)?
 (?P<minimumwidth>(?!0)\d+)?
 (?P<thousands_sep>,)?
-(?:\.(?P<precision>0|(?!0)\d+))?
+(?:\.
+    (?=\d|[,_])  # lookahead for digit or separator
+    (?P<precision>0|(?!0)\d+)?
+    (?P<frac_separators>[,_])?
+)?
 (?P<type>[eEfFgGn%])?
 \Z
 """, re.VERBOSE|re.DOTALL)
@@ -6192,6 +6196,9 @@ def _parse_format_specifier(format_spec, _localeconv=None):
         format_dict['grouping'] = [3, 0]
         format_dict['decimal_point'] = '.'
 
+    if format_dict['frac_separators'] is None:
+        format_dict['frac_separators'] = ''
+
     return format_dict
 
 def _format_align(sign, body, spec):
@@ -6311,6 +6318,11 @@ def _format_number(is_negative, intpart, fracpart, exp, spec):
 
     sign = _format_sign(is_negative, spec)
 
+    frac_sep = spec['frac_separators']
+    if fracpart and frac_sep:
+        fracpart = frac_sep.join(fracpart[pos:pos + 3]
+                                 for pos in range(0, len(fracpart), 3))
+
     if fracpart or spec['alt']:
         fracpart = spec['decimal_point'] + fracpart
 
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index d2327d247fa498..0feb320a7fb777 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -1082,6 +1082,15 @@ def test_formatting(self):
             (',%', '123.456789', '12,345.6789%'),
             (',e', '123456', '1.23456e+5'),
             (',E', '123456', '1.23456E+5'),
+            # and now for something completely different...
+            ('.,', '1.23456789', '1.234,567,89'),
+            ('._', '1.23456789', '1.234_567_89'),
+            ('.6_f', '1.23456789', '1.234_568'),
+            (',._%', '123.456789', '12,345.678_9%'),
+            (',._e', '123456', '1.234_56e+5'),
+            (',.4_e', '123456', '1.234_6e+5'),
+            (',.3_e', '123456', '1.235e+5'),
+            (',._E', '123456', '1.234_56E+5'),
 
             # negative zero: default behavior
             ('.1f', '-0', '-0.0'),
@@ -1155,6 +1164,9 @@ def test_formatting(self):
         # bytes format argument
         self.assertRaises(TypeError, Decimal(1).__format__, b'-020')
 
+        # precision or fractional part separator should follow after dot
+        self.assertRaises(ValueError, format, Decimal(1), '.f')
+
     def test_negative_zero_format_directed_rounding(self):
         with self.decimal.localcontext() as ctx:
             ctx.rounding = ROUND_CEILING
diff --git a/Misc/NEWS.d/next/Library/2025-04-07-09-53-54.gh-issue-87790.6nj3zQ.rst b/Misc/NEWS.d/next/Library/2025-04-07-09-53-54.gh-issue-87790.6nj3zQ.rst
new file mode 100644
index 00000000000000..cf80c71271bbd1
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-04-07-09-53-54.gh-issue-87790.6nj3zQ.rst
@@ -0,0 +1,2 @@
+Support underscore and comma as thousands separators in the fractional part
+for :class:`~decimal.Decimal`'s formatting.  Patch by Sergey B Kirpichev.