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

Doc/library/string.rst

@@ -319,14 +319,19 @@ non-empty format specification typically modifies the result.
 The general form of a *standard format specifier* is:
 
 .. productionlist:: format-spec
- format_spec: [[`fill`]`align`][`sign`]["z"]["#"]["0"][`width`][`grouping_option`]["." `precision`][`type`]
+ format_spec: [`options`][`width_and_precision`][`type`]
+ options: [[`fill`]`align`][`sign`]["z"]["#"]["0"]
  fill: <any character>
  align: "<" | ">" | "=" | "^"
  sign: "+" | "-" | " "
+ width_and_precision: [`width_with_grouping`][`precision_with_grouping`]
+ width_with_grouping: [`width`][`grouping_option`]
+ precision_with_grouping: "." [`precision`]`grouping_option`
  width: `~python-grammar:digit`+
  grouping_option: "_" | ","
  precision: `~python-grammar:digit`+
- type: "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
+ type: "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g"
+ : | "G" | "n" | "o" | "s" | "x" | "X" | "%"
 
 If a valid *align* value is specified, it can be preceded by a *fill*
 character that can be any character and defaults to a space if omitted.
@@ -458,6 +463,13 @@ indicates the maximum field size - in other words, how many characters will be
 used from the field content. The *precision* is not allowed for integer
 presentation types.
 
+The ``'_'`` or ``','`` option after *precision* means the use of an underscore
+or a comma for a thousands separator of the fractional part for floating-point
+presentation types.
+
+.. versionchanged:: 3.14
+ Support thousands separators for the fractional part.
+
 Finally, the *type* determines how the data should be presented.
 
 The available string presentation types are:
@@ -704,10 +716,18 @@ Replacing ``%x`` and ``%o`` and converting the value to different bases::
  >>> "int:{0:d}; hex:{0:#x}; oct:{0:#o}; bin:{0:#b}".format(42)
  'int: 42; hex: 0x2a; oct: 0o52; bin: 0b101010'
 
-Using the comma as a thousands separator::
+Using the comma or the underscore as a thousands separator::
 
  >>> '{:,}'.format(1234567890)
  '1,234,567,890'
+ >>> '{:_}'.format(1234567890)
+ '1_234_567_890'
+ >>> '{:_}'.format(123456789.123456789)
+ '123_456_789.12345679'
+ >>> '{:._}'.format(123456789.123456789)
+ '123456789.123_456_79'
+ >>> '{:_._}'.format(123456789.123456789)
+ '123_456_789.123_456_79'
 
 Expressing a percentage::
 

Doc/whatsnew/3.14.rst

@@ -278,6 +278,11 @@ Other language changes
  making it a :term:`generic type`.
  (Contributed by Brian Schubert in :gh:`126012`.)
 
+* Support underscore and comma as thousands separators in the fractional part
+ for floating-point presentation types of the new-style string formatting
+ (with :func:`format` or :ref:`f-strings`).
+ (Contrubuted by Sergey B Kirpichev in :gh:`87790`.)
+
 * ``\B`` in :mod:`regular expression <re>` now matches empty input string.
  Now it is always the opposite of ``\b``.
  (Contributed by Serhiy Storchaka in :gh:`124130`.)

Include/internal/pycore_unicodeobject.h

@@ -246,7 +246,8 @@ extern Py_ssize_t _PyUnicode_InsertThousandsGrouping(
  Py_ssize_t min_width,
  const char *grouping,
  PyObject *thousands_sep,
- Py_UCS4 *maxchar);
+ Py_UCS4 *maxchar,
+ int forward);
 
 /* --- Misc functions ----------------------------------------------------- */
 

Lib/test/test_float.py

@@ -754,6 +754,28 @@ def test_format(self):
  self.assertEqual(format(INF, 'f'), 'inf')
  self.assertEqual(format(INF, 'F'), 'INF')
 
+ # thousands separators
+ x = 123_456.123_456
+ self.assertEqual(format(x, '_f'), '123_456.123456')
+ self.assertEqual(format(x, ',f'), '123,456.123456')
+ self.assertEqual(format(x, '._f'), '123456.123_456')
+ self.assertEqual(format(x, '.,f'), '123456.123,456')
+ self.assertEqual(format(x, '_._f'), '123_456.123_456')
+ self.assertEqual(format(x, ',.,f'), '123,456.123,456')
+ self.assertEqual(format(x, '.10_f'), '123456.123_456_000_0')
+ self.assertEqual(format(x, '.10,f'), '123456.123,456,000,0')
+ self.assertEqual(format(x, '>21._f'), ' 123456.123_456')
+ self.assertEqual(format(x, '<21._f'), '123456.123_456 ')
+ self.assertEqual(format(x, '+.11_e'), '+1.234_561_234_56e+05')
+ self.assertEqual(format(x, '+.11,e'), '+1.234,561,234,56e+05')
+
+ self.assertRaises(ValueError, format, x, '._6f')
+ self.assertRaises(ValueError, format, x, '.,_f')
+ self.assertRaises(ValueError, format, x, '.6,_f')
+ self.assertRaises(ValueError, format, x, '.6_,f')
+ self.assertRaises(ValueError, format, x, '.6_n')
+ self.assertRaises(ValueError, format, x, '.6,n')
+
  @support.requires_IEEE_754
  def test_format_testfile(self):
  with open(format_testfile, encoding="utf-8") as testfile:

Lib/test/test_format.py

@@ -515,11 +515,15 @@ def test_with_a_commas_and_an_underscore_in_format_specifier(self):
  error_msg = re.escape("Cannot specify both ',' and '_'.")
  with self.assertRaisesRegex(ValueError, error_msg):
  '{:,_}'.format(1)
+ with self.assertRaisesRegex(ValueError, error_msg):
+ '{:.,_f}'.format(1.1)
 
  def test_with_an_underscore_and_a_comma_in_format_specifier(self):
  error_msg = re.escape("Cannot specify both ',' and '_'.")
  with self.assertRaisesRegex(ValueError, error_msg):
  '{:_,}'.format(1)
+ with self.assertRaisesRegex(ValueError, error_msg):
+ '{:._,f}'.format(1.1)
 
  def test_better_error_message_format(self):
  # https://bugs.python.org/issue20524

Misc/NEWS.d/next/Core_and_Builtins/2024-10-11-10-41-05.gh-issue-87790.mlfEGl.rst

@@ -0,0 +1,3 @@
+Support underscore and comma as thousands separators in the fractional part for
+floating-point presentation types of the new-style string formatting (with
+:func:`format` or :ref:`f-strings`). Patch by Sergey B Kirpichev.

Objects/stringlib/localeutil.h

@@ -47,7 +47,7 @@ InsertThousandsGrouping_fill(_PyUnicodeWriter *writer, Py_ssize_t *buffer_pos,
  PyObject *digits, Py_ssize_t *digits_pos,
  Py_ssize_t n_chars, Py_ssize_t n_zeros,
  PyObject *thousands_sep, Py_ssize_t thousands_sep_len,
- Py_UCS4 *maxchar)
+ Py_UCS4 *maxchar, int forward)
 {
  if (!writer){
  /* if maxchar > 127, maxchar is already set */
@@ -59,24 +59,39 @@ InsertThousandsGrouping_fill(_PyUnicodeWriter *writer, Py_ssize_t *buffer_pos,
  }
 
  if (thousands_sep){
- *buffer_pos -= thousands_sep_len;
-
+ if (!forward){
+ *buffer_pos -= thousands_sep_len;
+ }
  /* Copy the thousands_sep chars into the buffer. */
  _PyUnicode_FastCopyCharacters(writer->buffer, *buffer_pos,
  thousands_sep, 0,
  thousands_sep_len);
+ if (forward){
+ *buffer_pos += thousands_sep_len;
+ }
  }
 
- *buffer_pos -= n_chars;
- *digits_pos -= n_chars;
+ if (!forward){
+ *buffer_pos -= n_chars;
+ *digits_pos -= n_chars;
+ }
  _PyUnicode_FastCopyCharacters(writer->buffer, *buffer_pos,
  digits, *digits_pos,
  n_chars);
+ if (forward){
+ *buffer_pos += n_chars;
+ *digits_pos += n_chars;
+ }
 
  if (n_zeros){
- *buffer_pos -= n_zeros;
+ if (!forward){
+ *buffer_pos -= n_zeros;
+ }
  int kind = PyUnicode_KIND(writer->buffer);
  void *data = PyUnicode_DATA(writer->buffer);
  unicode_fill(kind, data, '0', *buffer_pos, n_zeros);
+ if (forward){
+ *buffer_pos += n_zeros;
+ }
  }
 }

Objects/unicodeobject.c

@@ -9764,7 +9764,8 @@ _PyUnicode_InsertThousandsGrouping(
  Py_ssize_t min_width,
  const char *grouping,
  PyObject *thousands_sep,
- Py_UCS4 *maxchar)
+ Py_UCS4 *maxchar,
+ int forward)
 {
  min_width = Py_MAX(0, min_width);
  if (writer){
@@ -9801,14 +9802,14 @@ _PyUnicode_InsertThousandsGrouping(
  should be an empty string */
  assert(!(grouping[0] == CHAR_MAX && thousands_sep_len != 0));
 
- digits_pos = d_pos + n_digits;
+ digits_pos = d_pos + (forward ? 0 : n_digits);
  if (writer){
- buffer_pos = writer->pos + n_buffer;
+ buffer_pos = writer->pos + (forward ? 0 : n_buffer);
  assert(buffer_pos <= PyUnicode_GET_LENGTH(writer->buffer));
  assert(digits_pos <= PyUnicode_GET_LENGTH(digits));
  }
  else{
- buffer_pos = n_buffer;
+ buffer_pos = forward ? 0 : n_buffer;
  }
 
  if (!writer){
@@ -9830,7 +9831,7 @@ _PyUnicode_InsertThousandsGrouping(
  digits, &digits_pos,
  n_chars, n_zeros,
  use_separator ? thousands_sep : NULL,
- thousands_sep_len, maxchar);
+ thousands_sep_len, maxchar, forward);
 
  /* Use a separator next time. */
  use_separator = 1;
@@ -9859,7 +9860,7 @@ _PyUnicode_InsertThousandsGrouping(
  digits, &digits_pos,
  n_chars, n_zeros,
  use_separator ? thousands_sep : NULL,
- thousands_sep_len, maxchar);
+ thousands_sep_len, maxchar, forward);
  }
  return count;
 }