gh-87790: support thousands separators for formatting fractional part of Decimal's

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
 

Lib/test/test_decimal.py

@@ -1088,6 +1088,15 @@ def test_formatting(self):
             ('07_', '1234.56', '1_234.56'),
             ('_', '1.23456789', '1.23456789'),
             ('_%', '123.456789', '12_345.6789%'),
+            # 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'),
@@ -1161,6 +1170,9 @@ 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

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.