Fix UniqueList membership after indexed replacement

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Keep `UniqueList` membership in sync when replacing an indexed item, and
+  leave membership unchanged when the index is out of range.
+
 ## 4.0.1 - 2026-08-30
 
 - Allow `uv_build` 0.12.x, contributed by @felixonmars in PR #49.

_python_utils_tests/test_containers.py

@@ -78,3 +78,47 @@ def test_sliceable_deque_eq() -> None:
     assert d == {1, 2, 3}
     assert d == d
     assert d == containers.SliceableDeque([1, 2, 3])
+
+
+@pytest.mark.parametrize('on_duplicate', ['ignore', 'raise'])
+@pytest.mark.parametrize('index', [0, -2])
+def test_unique_list_replace_membership(
+    on_duplicate: containers.OnDuplicate, index: int
+) -> None:
+    """Release the old value after replacing an indexed item."""
+    values = containers.UniqueList(1, 2, on_duplicate=on_duplicate)
+    values[index] = 3
+
+    assert values == [3, 2]
+    assert 1 not in values
+    assert 3 in values
+    values.append(1)
+    assert values == [3, 2, 1]
+
+
+@pytest.mark.parametrize('on_duplicate', ['ignore', 'raise'])
+@pytest.mark.parametrize('index', [2, -3])
+def test_unique_list_failed_replace_preserves_membership(
+    on_duplicate: containers.OnDuplicate, index: int
+) -> None:
+    """Do not reserve a value when indexed assignment fails."""
+    values = containers.UniqueList(1, 2, on_duplicate=on_duplicate)
+    with pytest.raises(IndexError):
+        values[index] = 3
+
+    assert values == [1, 2]
+    assert 3 not in values
+    values.append(3)
+    assert values == [1, 2, 3]
+
+
+@pytest.mark.parametrize('on_duplicate', ['ignore', 'raise'])
+def test_unique_list_replace_same_value(
+    on_duplicate: containers.OnDuplicate,
+) -> None:
+    """Replacing an item with itself keeps membership intact."""
+    values = containers.UniqueList(1, 2, on_duplicate=on_duplicate)
+    values[0] = 1
+    assert values == [1, 2]
+    assert 1 in values
+    assert 2 in values

python_utils/containers.py

@@ -494,17 +494,19 @@ def __setitem__(
             self._set.update(values)
         else:
             values = typing.cast(HT, values)
-            if values in self._set and values != self[indices]:
+            old_value = self[indices]
+            if values in self._set and values != old_value:
                 if self.on_duplicate == 'raise':
                     raise ValueError(f'Duplicate value: {values}')
                 else:
                     return
 
+            super().__setitem__(indices, values)
+            self._set.remove(old_value)
             self._set.add(values)
+            return
 
-        super().__setitem__(
-            typing.cast(slice, indices), typing.cast(list[HT], values)
-        )
+        super().__setitem__(indices, typing.cast(list[HT], values))
 
     def __delitem__(self, index: typing.SupportsIndex | slice) -> None:
         """