gh-61449: handle 0-padding option for Fractions like for floats

Author: skirpichev

Doc/library/fractions.rst

@@ -233,8 +233,7 @@ a single number, or from a string.
       :ref:`format specification mini-language <formatspec>`. The "alternate
       form" flag ``'#'`` is supported: if present, it forces the output string
       to always include an explicit denominator, even when the value being
-      formatted is an exact integer. The zero-fill flag ``'0'`` is not
-      supported.
+      formatted is an exact integer.
 
       If the ``format_spec`` format specification string ends with one of
       the presentation types ``'e'``, ``'E'``, ``'f'``, ``'F'``, ``'g'``,

Lib/fractions.py

@@ -500,6 +500,15 @@ def _format_float_style(self, match):
         no_neg_zero = bool(match["no_neg_zero"])
         alternate_form = bool(match["alt"])
         zeropad = bool(match["zeropad"])
+
+        # Support zeropad handling like built-in types (see gh-61449).
+        if match["zeropad"] is not None:
+            if match["fill"] is not None and match["align"] is not None:
+                zeropad = False
+            elif match["align"] is not None:
+                zeropad = False
+                fill = "0"
+
         minimumwidth = int(match["minimumwidth"] or "0")
         thousands_sep = match["thousands_sep"]
         precision = int(match["precision"] or "6")
@@ -603,10 +612,7 @@ def __format__(self, format_spec, /):
             return self._format_general(match)
 
         if match := _FLOAT_FORMAT_SPECIFICATION_MATCHER(format_spec):
-            # Refuse the temptation to guess if both alignment _and_
-            # zero padding are specified.
-            if match["align"] is None or match["zeropad"] is None:
-                return self._format_float_style(match)
+            return self._format_float_style(match)
 
         raise ValueError(
             f"Invalid format specifier {format_spec!r} "

Lib/test/test_fractions.py

@@ -1326,6 +1326,8 @@ def test_format_e_presentation_type(self):
             (F('1234567.123456'), '.5_e', '1.234_57e+06'),
             # z flag is legal, but never makes a difference to the output
             (F(-1, 7**100), 'z.6e', '-3.091690e-85'),
+            # fill and/or alignment with zero padding
+            (F(2, 3), '=010e', format(float(F(2, 3)), '=010e')),
         ]
         for fraction, spec, expected in testcases:
             with self.subTest(fraction=fraction, spec=spec):
@@ -1525,6 +1527,23 @@ def test_format_f_presentation_type(self):
             (F(151, 1000), '.1f', '0.2'),
             (F(22, 7), '.02f', '3.14'),  # issue gh-130662
             (F(22, 7), '005.02f', '03.14'),
+            # fill and/or alignment with zero padding
+            (F(2, 3), 'X>010f', format(float(F(2, 3)), 'X>010f')),
+            (F(2, 3), 'X<010f', format(float(F(2, 3)), 'X<010f')),
+            (F(2, 3), 'X^010f', format(float(F(2, 3)), 'X^010f')),
+            (F(2, 3), 'X=010f', format(float(F(2, 3)), 'X=010f')),
+            (F(2, 3), '0>010f', format(float(F(2, 3)), '0>010f')),
+            (F(2, 3), '0<010f', format(float(F(2, 3)), '0<010f')),
+            (F(2, 3), '0^010f', format(float(F(2, 3)), '0^010f')),
+            (F(2, 3), '0=010f', format(float(F(2, 3)), '0=010f')),
+            (F(2, 3), '>010f', format(float(F(2, 3)), '>010f')),
+            (F(2, 3), '<010f', format(float(F(2, 3)), '<010f')),
+            (F(2, 3), '^010f', format(float(F(2, 3)), '^010f')),
+            (F(2, 3), '=010e', format(float(F(2, 3)), '=010e')),
+            (F(2, 3), '=010f', format(float(F(2, 3)), '=010f')),
+            (F(2, 3), '>00.2f', format(float(F(2, 3)), '>00.2f')),
+            (F(2, 3), '>00f', format(float(F(2, 3)), '>00f')),
+            (F(2, 3), '=010%', format(float(F(2, 3)), '=010%')),
         ]
         for fraction, spec, expected in testcases:
             with self.subTest(fraction=fraction, spec=spec):
@@ -1593,6 +1612,8 @@ def test_format_g_presentation_type(self):
             (F(2**64), '_.25g', '18_446_744_073_709_551_616'),
             # As with 'e' format, z flag is legal, but has no effect
             (F(-1, 7**100), 'zg', '-3.09169e-85'),
+            # fill and/or alignment with zero padding
+            (F(2, 3), '=010g', format(float(F(2, 3)), '=010g')),
         ]
         for fraction, spec, expected in testcases:
             with self.subTest(fraction=fraction, spec=spec):
@@ -1605,24 +1626,6 @@ def test_invalid_formats(self):
 
         invalid_specs = [
             'Q6f',  # regression test
-            # illegal to use fill or alignment when zero padding
-            'X>010f',
-            'X<010f',
-            'X^010f',
-            'X=010f',
-            '0>010f',
-            '0<010f',
-            '0^010f',
-            '0=010f',
-            '>010f',
-            '<010f',
-            '^010f',
-            '=010e',
-            '=010f',
-            '=010g',
-            '=010%',
-            '>00.2f',
-            '>00f',
             # Missing precision
             '.e',
             '.f',

Misc/NEWS.d/next/Library/2026-03-10-19-30-39.gh-issue-61449.c6LcM0.rst

@@ -0,0 +1,2 @@
+Align support of zero-padding option for :class:`~fractions.Fraction`
+formatting with builtin numeric types.  Patch by Sergey B Kirpichev.