bpo-45708: Support underscore separators when formatting Decimal objects

Doc/library/decimal.rst

@@ -403,6 +403,11 @@ Decimal objects
       Underscores are allowed for grouping, as with integral and floating-point
       literals in code.
 
+   .. versionchanged:: 3.11
+      The underscore grouping option in the :ref:`formatting mini-language
+      <formatspec>` is now supported for :class:`Decimal` objects:
+      ``f"{Decimal(1234567):_}"`` gives ``'1_234_567'``.
+
    Decimal floating point objects share many properties with the other built-in
    numeric types such as :class:`float` and :class:`int`.  All of the usual math
    operations and special methods apply.  Likewise, decimal objects can be

Doc/whatsnew/3.11.rst

@@ -190,6 +190,15 @@ New Modules
 Improved Modules
 ================
 
+decimal
+-------
+
+* Formatting of :class:`~decimal.Decimal` objects now supports underscores
+  for grouping, as outlined in :PEP:`515`. For example,
+  ``f"{Decimal(1234567):_}"`` gives ``'1_234_567'``. Previously, only commas
+  were supported for grouping. (Contributed by Mark Dickinson in
+  :issue:`45708`.)
+
 fractions
 ---------
 

Lib/_pydecimal.py

@@ -6154,7 +6154,7 @@ def _convert_for_comparison(self, other, equality_op=False):
 (?P<alt>\#)?
 (?P<zeropad>0)?
 (?P<minimumwidth>(?!0)\d+)?
-(?P<thousands_sep>,)?
+(?P<thousands_sep>[_,])?
 (?:\.(?P<precision>0|(?!0)\d+))?
 (?P<type>[eEfFgGn%])?
 \Z

Lib/test/test_decimal.py

@@ -1050,7 +1050,7 @@ def test_formatting(self):
             ('\x00>10', '1.2345', '\x00\x00\x00\x001.2345'),
             ('\x00<10', '1.2345', '1.2345\x00\x00\x00\x00'),
 
-            # thousands separator
+            # thousands separator: ','
             (',', '1234567', '1,234,567'),
             (',', '123456', '123,456'),
             (',', '12345', '12,345'),
@@ -1082,6 +1082,38 @@ def test_formatting(self):
             (',e', '123456', '1.23456e+5'),
             (',E', '123456', '1.23456E+5'),
 
+            # thousands separator: '_'
+            ('_', '1234567', '1_234_567'),
+            ('_', '123456', '123_456'),
+            ('_', '12345', '12_345'),
+            ('_', '1234', '1_234'),
+            ('_', '123', '123'),
+            ('_', '12', '12'),
+            ('_', '1', '1'),
+            ('_', '0', '0'),
+            ('_', '-1234567', '-1_234_567'),
+            ('_', '-123456', '-123_456'),
+            ('7_', '123456', '123_456'),
+            ('8_', '123456', ' 123_456'),
+            ('08_', '123456', '0_123_456'), # special case: extra 0 needed
+            ('+08_', '123456', '+123_456'), # but not if there's a sign
+            (' 08_', '123456', ' 123_456'),
+            ('08_', '-123456', '-123_456'),
+            ('+09_', '123456', '+0_123_456'),
+            # ... with fractional part...
+            ('07_', '1234.56', '1_234.56'),
+            ('08_', '1234.56', '1_234.56'),
+            ('09_', '1234.56', '01_234.56'),
+            ('010_', '1234.56', '001_234.56'),
+            ('011_', '1234.56', '0_001_234.56'),
+            ('012_', '1234.56', '0_001_234.56'),
+            ('08_.1f', '1234.5', '01_234.5'),
+            # no thousands separators in fraction part
+            ('_', '1.23456789', '1.23456789'),
+            ('_%', '123.456789', '12_345.6789%'),
+            ('_e', '123456', '1.23456e+5'),
+            ('_E', '123456', '1.23456E+5'),
+
             # issue 6850
             ('a=-7.0', '0.12345', 'aaaa0.1'),
 

Misc/NEWS.d/next/Library/2021-11-06-10-33-43.bpo-45708.LEc5wE.rst

@@ -0,0 +1,3 @@
+Support underscores in :class:`~decimal.Decimal` object formatting: for
+example, ``f"{Decimal(1234567):_}"`` now gives ``'1_234_567'``. (Previously,
+this gave an "invalid format string" :exc:`ValueError`.)

Modules/_decimal/libmpdec/io.c

@@ -861,6 +861,12 @@ mpd_parse_fmt_str(mpd_spec_t *spec, const char *fmt, int caps)
         spec->grouping = "\003\003";
         cp++;
     }
+    else if (*cp == '_') {
+        spec->dot = ".";
+        spec->sep = "_";
+        spec->grouping = "\003\003";
+        cp++;
+    }
 
     /* fraction digits or significant digits */
     if (*cp == '.') {