gh-130664: Treat '0' fill character with align '=' as zero-padding for Fraction's (GH-131067)

(cherry picked from commit c113a8e)

Co-authored-by: Sergey B Kirpichev <[email protected]>

Author: skirpichev

Lib/fractions.py

@@ -481,6 +481,9 @@ def _format_float_style(self, match):
         trim_point = not alternate_form
         exponent_indicator = "E" if presentation_type in "EFG" else "e"
 
+        if align == '=' and fill == '0':
+            zeropad = True
+
         # Round to get the digits we need, figure out where to place the point,
         # and decide whether to use scientific notation. 'point_pos' is the
         # relative to the _end_ of the digit string: that is, it's the number

Lib/test/test_fractions.py

@@ -1411,11 +1411,8 @@ def test_format_f_presentation_type(self):
             (F('-1234.5678'), '08,.0f', '-001,235'),
             (F('-1234.5678'), '09,.0f', '-0,001,235'),
             # Corner-case - zero-padding specified through fill and align
-            # instead of the zero-pad character - in this case, treat '0' as a
-            # regular fill character and don't attempt to insert commas into
-            # the filled portion. This differs from the int and float
-            # behaviour.
-            (F('1234.5678'), '0=12,.2f', '00001,234.57'),
+            # instead of the zero-pad character.
+            (F('1234.5678'), '0=12,.2f', '0,001,234.57'),
             # Corner case where it's not clear whether the '0' indicates zero
             # padding or gives the minimum width, but there's still an obvious
             # answer to give. We want this to work in case the minimum width

Misc/NEWS.d/next/Library/2025-03-11-05-24-14.gh-issue-130664.g0yNMm.rst

@@ -0,0 +1,4 @@
+Handle corner-case for :class:`~fractions.Fraction`'s formatting: treat
+zero-padding (preceding the width field by a zero (``'0'``) character) as an
+equivalent to a fill character of ``'0'`` with an alignment type of ``'='``,
+just as in case of :class:`float`'s.