From 1d03e6da95443ddb7b7d932d175dd3f8806eba92 Mon Sep 17 00:00:00 2001
From: AryanRG <aryanrganjoo@gmail.com>
Date: Wed, 14 Oct 2026 15:52:21 +0530
Subject: [PATCH] Fix fallback for partial list patterns

Resolve missing list-pattern parts individually without mutating locale
data. Follow the CLDR root chain for unit-narrow so narrow lists do not
fall back to unrelated wide-unit parts.

Cover partial start/end patterns, missing two-item patterns, precedence,
and locale-cache preservation with regressions.
---
 babel/lists.py      | 22 ++++++++++++++--------
 tests/test_lists.py | 37 ++++++++++++++++++++++++++++++++++++-
 2 files changed, 50 insertions(+), 9 deletions(-)

diff --git a/babel/lists.py b/babel/lists.py
index b6c859800..049905a9d 100644
--- a/babel/lists.py
+++ b/babel/lists.py
@@ -126,16 +126,22 @@ def format_list(
     "standard-narrow": ["standard-short", "standard"],
     "standard-short": ["standard"],
     "unit": ["unit-short", "standard"],
-    "unit-narrow": ["unit-short", "unit", "standard"],
+    "unit-narrow": ["unit-short", "standard"],
     "unit-short": ["standard"],
 }
 
 
 def _resolve_list_style(locale: Locale, style: str):
-    for style in (style, *(_style_fallbacks.get(style, []))):  # noqa: B020
-        if style in locale.list_patterns:
-            return locale.list_patterns[style]
-    raise ValueError(
-        f"Locale {locale} does not support list formatting style {style!r} "
-        f"(supported are {sorted(locale.list_patterns)})",
-    )
+    # A style may define only some of its parts, so missing parts are
+    # filled in from the fallback styles, in order.
+    patterns = {}
+    for fallback_style in (style, *(_style_fallbacks.get(style, []))):
+        if fallback_style in locale.list_patterns:
+            for part, pattern in locale.list_patterns[fallback_style].items():
+                patterns.setdefault(part, pattern)
+    if not patterns:
+        raise ValueError(
+            f"Locale {locale} does not support list formatting style {style!r} "
+            f"(supported are {sorted(locale.list_patterns)})",
+        )
+    return patterns
diff --git a/tests/test_lists.py b/tests/test_lists.py
index 448366076..32c82bcc5 100644
--- a/tests/test_lists.py
+++ b/tests/test_lists.py
@@ -1,6 +1,6 @@
 import pytest
 
-from babel import lists, units
+from babel import Locale, lists, units
 
 
 @pytest.mark.parametrize(
@@ -43,3 +43,38 @@ def test_issue_1098():
 def test_lists_default_locale_deprecation():
     with pytest.warns(DeprecationWarning):
         _ = lists.DEFAULT_LOCALE
+
+
+@pytest.mark.parametrize(
+    ('list', 'style', 'locale', 'expected'),
+    [
+        # Styles defining only some parts get the missing ones from fallback styles.
+        (['A', 'B', 'C'], 'unit', 'de', 'A, B und C'),
+        (['A', 'B', 'C'], 'standard-narrow', 'fr', 'A, B, C'),
+        (['A', 'B', 'C', 'D'], 'standard-narrow', 'fr', 'A, B, C, D'),
+        (['A', 'B', 'C'], 'unit', 'hy', 'A, B և C'),
+        (['A', 'B', 'C'], 'unit-narrow', 'is', 'A B og C'),
+        (['A', 'B', 'C'], 'unit', 'ar_OM', 'A، وB، وC'),
+        # The nearest fallback style wins: 'end' from standard-short, 'start' from standard.
+        (['A', 'B', 'C'], 'standard-narrow', 'wo_SN', 'A, B, & C'),
+        # Two items with no '2' part use the fallback '2' pattern.
+        (['A', 'B'], 'standard-short', 'hi', 'A और B'),
+        (['A', 'B'], 'standard-narrow', 'lo', 'A ແລະ B'),
+        (['A', 'B'], 'unit-narrow', 'wo_SN', 'A, B'),
+        (['A', 'B'], 'unit-narrow', 'sat', 'A, B'),
+        (['A', 'B'], 'unit', 'ar_OM', 'A وB'),
+        # Parts defined by the requested style take precedence.
+        (['A', 'B', 'C'], 'standard-narrow', 'lo', 'A, B, C'),
+        (['A', 'B'], 'unit-short', 'de', 'A, B'),
+    ],
+)
+def test_format_list_partial_style_fallback(list, style, locale, expected):
+    assert lists.format_list(list, style=style, locale=locale) == expected
+
+
+def test_format_list_does_not_modify_locale_patterns():
+    locale = Locale.parse('fr')
+    before = {style: dict(parts) for style, parts in locale.list_patterns.items()}
+    lists.format_list(['A', 'B', 'C', 'D'], style='standard-narrow', locale=locale)
+    after = {style: dict(parts) for style, parts in locale.list_patterns.items()}
+    assert after == before