gh-87790: support thousands separators for formatting fractional part of floats (#125304)

```pycon
>>> f"{123_456.123_456:_._f}"  # Whole and fractional
'123_456.123_456'
>>> f"{123_456.123_456:_f}"    # Integer component only
'123_456.123456'
>>> f"{123_456.123_456:._f}"   # Fractional component only
'123456.123_456'
>>> f"{123_456.123_456:.4_f}"  # with precision
'123456.1_235'
```

9 files changed, 218 insertions, 45 deletions

diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 09165c4..721c5c8 100644
--- a/Doc/library/string.rst
+++ b/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::
 
diff --git a/Doc/whatsnew/3.14.rst b/Doc/whatsnew/3.14.rst
index 1cd8da4..dbd59a9 100644
--- a/Doc/whatsnew/3.14.rst
+++ b/Doc/whatsnew/3.14.rst
@@ -336,6 +336,11 @@ 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`.)
diff --git a/Include/internal/pycore_unicodeobject.h b/Include/internal/pycore_unicodeobject.h
index a60372f..13c3213 100644
--- a/Include/internal/pycore_unicodeobject.h
+++ b/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 ----------------------------------------------------- */
 
diff --git a/Lib/test/test_float.py b/Lib/test/test_float.py
index f588e16..231b104 100644
--- a/Lib/test/test_float.py
+++ b/Lib/test/test_float.py
@@ -754,6 +754,28 @@ class FormatTestCase(unittest.TestCase):
         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:
diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
index 9dde63e..3916bc3 100644
--- a/Lib/test/test_format.py
+++ b/Lib/test/test_format.py
@@ -515,11 +515,15 @@ class FormatTest(unittest.TestCase):
         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
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-10-11-10-41-05.gh-issue-87790.mlfEGl.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-10-11-10-41-05.gh-issue-87790.mlfEGl.rst
new file mode 100644
index 0000000..46a761f
--- /dev/null
+++ b/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.
diff --git a/Objects/stringlib/localeutil.h b/Objects/stringlib/localeutil.h
index d77715e..a4ab701 100644
--- a/Objects/stringlib/localeutil.h
+++ b/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;
+        }
     }
 }
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 03f15b3..0686276 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9772,7 +9772,8 @@ _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) {
@@ -9809,14 +9810,14 @@ _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) {
@@ -9838,7 +9839,7 @@ _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;
@@ -9867,7 +9868,7 @@ _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;
 }
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 16f7111..4f8662b 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -135,6 +135,7 @@ typedef struct {
     Py_ssize_t width;
     enum LocaleType thousands_separators;
     Py_ssize_t precision;
+    enum LocaleType frac_thousands_separator;
     Py_UCS4 type;
 } InternalFormatSpec;
 
@@ -171,6 +172,7 @@ parse_internal_render_format_spec(PyObject *obj,
     format->sign = '\0';
     format->width = -1;
     format->thousands_separators = LT_NO_LOCALE;
+    format->frac_thousands_separator = LT_NO_LOCALE;
     format->precision = -1;
     format->type = default_type;
 
@@ -260,7 +262,35 @@ parse_internal_render_format_spec(PyObject *obj,
             /* Overflow error. Exception already set. */
             return 0;
 
-        /* Not having a precision after a dot is an error. */
+        if (end-pos && READ_spec(pos) == ',') {
+            if (consumed == 0) {
+                format->precision = -1;
+            }
+            format->frac_thousands_separator = LT_DEFAULT_LOCALE;
+            ++pos;
+            ++consumed;
+        }
+        if (end-pos && READ_spec(pos) == '_') {
+            if (format->frac_thousands_separator != LT_NO_LOCALE) {
+                invalid_comma_and_underscore();
+                return 0;
+            }
+            if (consumed == 0) {
+                format->precision = -1;
+            }
+            format->frac_thousands_separator = LT_UNDERSCORE_LOCALE;
+            ++pos;
+            ++consumed;
+        }
+        if (end-pos && READ_spec(pos) == ',') {
+            if (format->frac_thousands_separator == LT_UNDERSCORE_LOCALE) {
+                invalid_comma_and_underscore();
+                return 0;
+            }
+        }
+
+        /* Not having a precision or underscore/comma after a dot
+           is an error. */
         if (consumed == 0) {
             PyErr_Format(PyExc_ValueError,
                          "Format specifier missing precision");
@@ -327,6 +357,14 @@ parse_internal_render_format_spec(PyObject *obj,
         }
     }
 
+    if (format->type == 'n'
+        && format->frac_thousands_separator != LT_NO_LOCALE)
+    {
+        invalid_thousands_separator_type(format->frac_thousands_separator,
+                                         format->type);
+        return 0;
+    }
+
     assert (format->align <= 127);
     assert (format->sign <= 127);
     return 1;
@@ -402,6 +440,7 @@ fill_padding(_PyUnicodeWriter *writer,
 typedef struct {
     PyObject *decimal_point;
     PyObject *thousands_sep;
+    PyObject *frac_thousands_sep;
     const char *grouping;
     char *grouping_buffer;
 } LocaleInfo;
@@ -423,6 +462,8 @@ typedef struct {
     Py_ssize_t n_remainder; /* Digits in decimal and/or exponent part,
                                excluding the decimal itself, if
                                present. */
+    Py_ssize_t n_frac;
+    Py_ssize_t n_grouped_frac_digits;
 
     /* These 2 are not the widths of fields, but are needed by
        STRINGLIB_GROUPING. */
@@ -445,24 +486,32 @@ typedef struct {
 */
 static void
 parse_number(PyObject *s, Py_ssize_t pos, Py_ssize_t end,
-             Py_ssize_t *n_remainder, int *has_decimal)
+             Py_ssize_t *n_remainder, Py_ssize_t *n_frac, int *has_decimal)
 {
-    Py_ssize_t remainder;
+    Py_ssize_t frac;
     int kind = PyUnicode_KIND(s);
     const void *data = PyUnicode_DATA(s);
 
-    while (pos<end && Py_ISDIGIT(PyUnicode_READ(kind, data, pos)))
+    while (pos<end && Py_ISDIGIT(PyUnicode_READ(kind, data, pos))) {
         ++pos;
-    remainder = pos;
+    }
+    frac = pos;
 
     /* Does remainder start with a decimal point? */
-    *has_decimal = pos<end && PyUnicode_READ(kind, data, remainder) == '.';
+    *has_decimal = pos<end && PyUnicode_READ(kind, data, frac) == '.';
 
     /* Skip the decimal point. */
-    if (*has_decimal)
-        remainder++;
+    if (*has_decimal) {
+        frac++;
+        pos++;
+    }
+
+    while (pos<end && Py_ISDIGIT(PyUnicode_READ(kind, data, pos))) {
+        ++pos;
+    }
 
-    *n_remainder = end - remainder;
+    *n_frac = pos - frac;
+    *n_remainder = end - pos;
 }
 
 /* not all fields of format are used.  for example, precision is
@@ -473,18 +522,19 @@ parse_number(PyObject *s, Py_ssize_t pos, Py_ssize_t end,
 static Py_ssize_t
 calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
                    Py_UCS4 sign_char, Py_ssize_t n_start,
-                   Py_ssize_t n_end, Py_ssize_t n_remainder,
+                   Py_ssize_t n_end, Py_ssize_t n_remainder, Py_ssize_t n_frac,
                    int has_decimal, const LocaleInfo *locale,
                    const InternalFormatSpec *format, Py_UCS4 *maxchar)
 {
     Py_ssize_t n_non_digit_non_padding;
     Py_ssize_t n_padding;
 
-    spec->n_digits = n_end - n_start - n_remainder - (has_decimal?1:0);
+    spec->n_digits = n_end - n_start - n_frac - n_remainder - (has_decimal?1:0);
     spec->n_lpadding = 0;
     spec->n_prefix = n_prefix;
     spec->n_decimal = has_decimal ? PyUnicode_GET_LENGTH(locale->decimal_point) : 0;
     spec->n_remainder = n_remainder;
+    spec->n_frac = n_frac;
     spec->n_spadding = 0;
     spec->n_rpadding = 0;
     spec->sign = '\0';
@@ -530,7 +580,7 @@ calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
 
     /* The number of chars used for non-digits and non-padding. */
     n_non_digit_non_padding = spec->n_sign + spec->n_prefix + spec->n_decimal +
-        spec->n_remainder;
+        + spec->n_frac + spec->n_remainder;
 
     /* min_width can go negative, that's okay. format->width == -1 means
        we don't care. */
@@ -550,19 +600,36 @@ calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
             NULL, 0,
             NULL, 0, spec->n_digits,
             spec->n_min_width,
-            locale->grouping, locale->thousands_sep, &grouping_maxchar);
+            locale->grouping, locale->thousands_sep, &grouping_maxchar, 0);
         if (spec->n_grouped_digits == -1) {
             return -1;
         }
         *maxchar = Py_MAX(*maxchar, grouping_maxchar);
     }
 
+    if (spec->n_frac == 0) {
+        spec->n_grouped_frac_digits = 0;
+    }
+    else {
+        Py_UCS4 grouping_maxchar;
+        spec->n_grouped_frac_digits = _PyUnicode_InsertThousandsGrouping(
+            NULL, 0,
+            NULL, 0, spec->n_frac,
+            spec->n_frac,
+            locale->grouping, locale->frac_thousands_sep, &grouping_maxchar, 1);
+        if (spec->n_grouped_frac_digits == -1) {
+            return -1;
+        }
+        *maxchar = Py_MAX(*maxchar, grouping_maxchar);
+    }
+
     /* Given the desired width and the total of digit and non-digit
        space we consume, see if we need any padding. format->width can
        be negative (meaning no padding), but this code still works in
        that case. */
     n_padding = format->width -
-                        (n_non_digit_non_padding + spec->n_grouped_digits);
+                        (n_non_digit_non_padding + spec->n_grouped_digits
+                         + spec->n_grouped_frac_digits - spec->n_frac);
     if (n_padding > 0) {
         /* Some padding is needed. Determine if it's left, space, or right. */
         switch (format->align) {
@@ -593,7 +660,7 @@ calc_number_widths(NumberFieldWidths *spec, Py_ssize_t n_prefix,
 
     return spec->n_lpadding + spec->n_sign + spec->n_prefix +
         spec->n_spadding + spec->n_grouped_digits + spec->n_decimal +
-        spec->n_remainder + spec->n_rpadding;
+        spec->n_grouped_frac_digits + spec->n_remainder + spec->n_rpadding;
 }
 
 /* Fill in the digit parts of a number's string representation,
@@ -649,7 +716,7 @@ fill_number(_PyUnicodeWriter *writer, const NumberFieldWidths *spec,
                 writer, spec->n_grouped_digits,
                 digits, d_pos, spec->n_digits,
                 spec->n_min_width,
-                locale->grouping, locale->thousands_sep, NULL);
+                locale->grouping, locale->thousands_sep, NULL, 0);
         if (r == -1)
             return -1;
         assert(r == spec->n_grouped_digits);
@@ -677,6 +744,19 @@ fill_number(_PyUnicodeWriter *writer, const NumberFieldWidths *spec,
         d_pos += 1;
     }
 
+    if (spec->n_frac) {
+        r = _PyUnicode_InsertThousandsGrouping(
+                writer, spec->n_grouped_frac_digits,
+                digits, d_pos, spec->n_frac, spec->n_frac,
+                locale->grouping, locale->frac_thousands_sep, NULL, 1);
+        if (r == -1) {
+            return -1;
+        }
+        assert(r == spec->n_grouped_frac_digits);
+        d_pos += spec->n_frac;
+        writer->pos += spec->n_grouped_frac_digits;
+    }
+
     if (spec->n_remainder) {
         _PyUnicode_FastCopyCharacters(
             writer->buffer, writer->pos,
@@ -701,7 +781,8 @@ static const char no_grouping[1] = {CHAR_MAX};
    LT_CURRENT_LOCALE, a hard-coded locale if LT_DEFAULT_LOCALE or
    LT_UNDERSCORE_LOCALE/LT_UNDER_FOUR_LOCALE, or none if LT_NO_LOCALE. */
 static int
-get_locale_info(enum LocaleType type, LocaleInfo *locale_info)
+get_locale_info(enum LocaleType type, enum LocaleType frac_type,
+                LocaleInfo *locale_info)
 {
     switch (type) {
     case LT_CURRENT_LOCALE: {
@@ -746,6 +827,19 @@ get_locale_info(enum LocaleType type, LocaleInfo *locale_info)
         locale_info->grouping = no_grouping;
         break;
     }
+    if (frac_type != LT_NO_LOCALE) {
+        locale_info->frac_thousands_sep = PyUnicode_FromOrdinal(
+            frac_type == LT_DEFAULT_LOCALE ? ',' : '_');
+        if (!locale_info->frac_thousands_sep) {
+            return -1;
+        }
+        if (locale_info->grouping == no_grouping) {
+            locale_info->grouping = "\3";
+        }
+    }
+    else {
+        locale_info->frac_thousands_sep = Py_GetConstant(Py_CONSTANT_EMPTY_STR);
+    }
     return 0;
 }
 
@@ -754,6 +848,7 @@ free_locale_info(LocaleInfo *locale_info)
 {
     Py_XDECREF(locale_info->decimal_point);
     Py_XDECREF(locale_info->thousands_sep);
+    Py_XDECREF(locale_info->frac_thousands_sep);
     PyMem_Free(locale_info->grouping_buffer);
 }
 
@@ -1005,13 +1100,13 @@ format_long_internal(PyObject *value, const InternalFormatSpec *format,
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        format->thousands_separators,
+                        format->thousands_separators, 0,
                         &locale) == -1)
         goto done;
 
     /* Calculate how much memory we'll need. */
     n_total = calc_number_widths(&spec, n_prefix, sign_char, inumeric_chars,
-                                 inumeric_chars + n_digits, n_remainder, 0,
+                                 inumeric_chars + n_digits, n_remainder, 0, 0,
                                  &locale, format, &maxchar);
     if (n_total == -1) {
         goto done;
@@ -1046,6 +1141,7 @@ format_float_internal(PyObject *value,
     char *buf = NULL;       /* buffer returned from PyOS_double_to_string */
     Py_ssize_t n_digits;
     Py_ssize_t n_remainder;
+    Py_ssize_t n_frac;
     Py_ssize_t n_total;
     int has_decimal;
     double val;
@@ -1125,7 +1221,8 @@ format_float_internal(PyObject *value,
     if (format->sign != '+' && format->sign != ' '
         && format->width == -1
         && format->type != 'n'
-        && !format->thousands_separators)
+        && !format->thousands_separators
+        && !format->frac_thousands_separator)
     {
         /* Fast path */
         result = _PyUnicodeWriter_WriteASCIIString(writer, buf, n_digits);
@@ -1151,18 +1248,20 @@ format_float_internal(PyObject *value,
 
     /* Determine if we have any "remainder" (after the digits, might include
        decimal or exponent or both (or neither)) */
-    parse_number(unicode_tmp, index, index + n_digits, &n_remainder, &has_decimal);
+    parse_number(unicode_tmp, index, index + n_digits,
+                 &n_remainder, &n_frac, &has_decimal);
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
                         format->thousands_separators,
+                        format->frac_thousands_separator,
                         &locale) == -1)
         goto done;
 
     /* Calculate how much memory we'll need. */
     n_total = calc_number_widths(&spec, 0, sign_char, index,
-                                 index + n_digits, n_remainder, has_decimal,
-                                 &locale, format, &maxchar);
+                                 index + n_digits, n_remainder, n_frac,
+                                 has_decimal, &locale, format, &maxchar);
     if (n_total == -1) {
         goto done;
     }
@@ -1202,6 +1301,8 @@ format_complex_internal(PyObject *value,
     Py_ssize_t n_im_digits;
     Py_ssize_t n_re_remainder;
     Py_ssize_t n_im_remainder;
+    Py_ssize_t n_re_frac;
+    Py_ssize_t n_im_frac;
     Py_ssize_t n_re_total;
     Py_ssize_t n_im_total;
     int re_has_decimal;
@@ -1330,13 +1431,14 @@ format_complex_internal(PyObject *value,
     /* Determine if we have any "remainder" (after the digits, might include
        decimal or exponent or both (or neither)) */
     parse_number(re_unicode_tmp, i_re, i_re + n_re_digits,
-                 &n_re_remainder, &re_has_decimal);
+                 &n_re_remainder, &n_re_frac, &re_has_decimal);
     parse_number(im_unicode_tmp, i_im, i_im + n_im_digits,
-                 &n_im_remainder, &im_has_decimal);
+                 &n_im_remainder, &n_im_frac, &im_has_decimal);
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
                         format->thousands_separators,
+                        format->frac_thousands_separator,
                         &locale) == -1)
         goto done;
 
@@ -1349,8 +1451,8 @@ format_complex_internal(PyObject *value,
     /* Calculate how much memory we'll need. */
     n_re_total = calc_number_widths(&re_spec, 0, re_sign_char,
                                     i_re, i_re + n_re_digits, n_re_remainder,
-                                    re_has_decimal, &locale, &tmp_format,
-                                    &maxchar);
+                                    n_re_frac, re_has_decimal, &locale,
+                                    &tmp_format, &maxchar);
     if (n_re_total == -1) {
         goto done;
     }
@@ -1362,8 +1464,8 @@ format_complex_internal(PyObject *value,
         tmp_format.sign = '+';
     n_im_total = calc_number_widths(&im_spec, 0, im_sign_char,
                                     i_im, i_im + n_im_digits, n_im_remainder,
-                                    im_has_decimal, &locale, &tmp_format,
-                                    &maxchar);
+                                    n_im_frac, im_has_decimal, &locale,
+                                    &tmp_format, &maxchar);
     if (n_im_total == -1) {
         goto done;
     }