Allow UniqueList self-assignment for non-reflexive values

_python_utils_tests/test_containers.py

@@ -1209,3 +1209,31 @@ def test_unique_list_failed_assignment_takes_membership_back() -> None:
     assert 1 in values
     values.append(3)
     assert values == [1, 2, 3]
+
+
+@pytest.mark.parametrize('index', [0, -1])
+@pytest.mark.parametrize('mode', ['ignore', 'raise'])
+def test_unique_list_nan_self_assignment(
+    index: int, mode: containers.OnDuplicate
+) -> None:
+    """Assigning the same NaN object does not introduce a duplicate."""
+    missing = float('nan')
+    values: containers.UniqueList[float] = containers.UniqueList(
+        missing, on_duplicate=mode
+    )
+    values[index] = missing
+    assert len(values) == 1
+    assert values[index] is missing
+    assert missing in values
+
+
+def test_unique_list_nan_duplicate_in_another_slot() -> None:
+    """The same NaN object remains a duplicate in a different list slot."""
+    missing = float('nan')
+    values: containers.UniqueList[float] = containers.UniqueList(
+        1.0, missing, on_duplicate='raise'
+    )
+    with pytest.raises(ValueError, match='Duplicate value'):
+        values[0] = missing
+    assert values[0] == 1.0
+    assert values[1] is missing

python_utils/containers.py

@@ -554,6 +554,15 @@ class UniqueList(list[HT]):
     >>> l
     [5, 10, 2, 3, 4]
 
+    Assigning an item to itself keeps it, including a NaN that does not
+    compare equal to itself:
+
+    >>> missing = float('nan')
+    >>> samples = UniqueList(missing, on_duplicate='raise')
+    >>> samples[0] = missing
+    >>> len(samples)
+    1
+
     A value that was removed or replaced can be added again:
 
     >>> l = UniqueList(1, 2, 3)
@@ -952,7 +961,7 @@ def _set_index(self, index: typing.SupportsIndex, value: HT) -> None:
         """
         old_value: HT = self[index]
         known: bool = value in self._set
-        if known and value != old_value:
+        if known and value is not old_value and value != old_value:
             if self.on_duplicate == 'raise':
                 raise ValueError(f'Duplicate value: {value}')
             else: