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

Lib/_pydecimal.py

@@ -6122,7 +6122,11 @@ def _convert_for_comparison(self, other, equality_op=False):
 (?P<zeropad>0)?
 (?P<minimumwidth>\d+)?
 (?P<thousands_sep>[,_])?
-(?:\.(?P<precision>\d+))?
+(?:\.
+ (?=[\d,_]) # lookahead for digit or separator
+ (?P<precision>\d+)?
+ (?P<frac_separators>[,_])?
+)?
 (?P<type>[eEfFgGn%])?
 \z
 """, re.VERBOSE|re.DOTALL)
@@ -6215,6 +6219,9 @@ 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):
@@ -6334,6 +6341,11 @@ 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

@@ -1089,6 +1089,15 @@ 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', '12345.23456789', '12345.234_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'),
@@ -1162,6 +1171,10 @@ 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')
+ self.assertRaises(ValueError, format, Decimal(1), '._6f')
+
  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.