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

Doc/library/string.rst

@@ -312,7 +312,7 @@ 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: [[`fill`]`align`][`sign`]["z"]["#"]["0"][`width`][`grouping_option`]["." `precision` [`grouping_option`]][`type`]
    fill: <any character>
    align: "<" | ">" | "=" | "^"
    sign: "+" | "-" | " "
@@ -449,6 +449,13 @@ 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:

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 +754,19 @@ def test_format(self):
         self.assertEqual(format(INF, 'f'), 'inf')
         self.assertEqual(format(INF, 'F'), 'INF')
 
+        # underscores
+        x = 123_456.123_456
+        self.assertEqual(format(x, '_f'), '123_456.123456')
+        self.assertEqual(format(x, '._f'), '123456.123_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, '>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')
+
     @support.requires_IEEE_754
     def test_format_testfile(self):
         with open(format_testfile, encoding="utf-8") as testfile:

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

@@ -9488,7 +9488,8 @@ _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) {
@@ -9525,14 +9526,14 @@ _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) {
@@ -9554,7 +9555,7 @@ _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;
@@ -9583,7 +9584,7 @@ _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;
 }