Preserve proxy set operations and relation assignments

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Relation components were incorrectly loaded from pickled data as pointing in the wrong direction.
+- Set manipulations for `Entity.tags` and `Entity.relation_tags_many` are now more reliable and will no longer drop data.
+  https://github.com/HexDecimal/python-tcod-ecs/pull/58
 
 ## [5.5.0] - 2026-01-14
 

tcod/ecs/entity.py

@@ -602,6 +602,11 @@ class EntityTags(MutableSet[Any]):
     entity: Entity
     traverse: tuple[object, ...]
 
+    @classmethod
+    def _from_iterable(cls, it: Iterable[_T1]) -> set[_T1]:
+        """Return an independent set for the inherited set operations."""
+        return set(it)
+
     def __call__(self, *, traverse: Iterable[object]) -> Self:
         """Update this view with alternative parameters, such as a specific traversal relation.
 
@@ -725,6 +730,11 @@ class EntityRelationsMapping(MutableSet[Entity]):
     key: object
     traverse: tuple[object, ...]
 
+    @classmethod
+    def _from_iterable(cls, it: Iterable[_T1]) -> set[_T1]:
+        """Return an independent set for the inherited set operations."""
+        return set(it)
+
     if __debug__:
 
         def __attrs_post_init__(self) -> None:
@@ -826,6 +836,7 @@ def __getitem__(self, key: object) -> EntityRelationsMapping:
     def __setitem__(self, key: object, values: Iterable[Entity]) -> None:
         """Overwrite the targets of a relation tag with the new values."""
         assert not isinstance(values, Entity), "Did you mean `entity.relations[key] = (target,)`?"
+        values = tuple(values)  # Values may be a live view of the mapping being replaced.
         mapping = EntityRelationsMapping(self.entity, key, self.traverse)
         mapping.clear()
         for v in values:

tests/test_relations.py

@@ -13,6 +13,92 @@
 ChildOf: Final = "ChildOf"
 
 
+@pytest.mark.parametrize("operation", ["union", "intersection", "difference", "symmetric_difference"])
+def test_inplace_relation_updates(operation: str) -> None:
+    world = tcod.ecs.Registry()
+    player = world["player"]
+    first, second, third = world["first"], world["second"], world["third"]
+    expected = {first, second}
+    other = {second, third}
+    player.relation_tags_many["allies"] = expected
+    query = world.Q.all_of(relations=[("allies", second)])
+    assert set(query) == {player}
+
+    if operation == "union":
+        expected |= other
+        player.relation_tags_many["allies"] |= other
+    elif operation == "intersection":
+        expected &= other
+        player.relation_tags_many["allies"] &= other
+    elif operation == "difference":
+        expected -= other
+        player.relation_tags_many["allies"] -= other
+    else:
+        expected ^= other
+        player.relation_tags_many["allies"] ^= other
+
+    assert set(player.relation_tags_many["allies"]) == expected
+    assert set(query) == ({player} if second in expected else set())
+    assert set(world.Q.all_of(relations=[(player, "allies", None)])) == expected
+
+
+def test_relation_assignment_from_lazy_self_view() -> None:
+    world = tcod.ecs.Registry()
+    player = world["player"]
+    first, second = world["first"], world["second"]
+    player.relation_tags_many["allies"] = (first, second)
+
+    player.relation_tags_many["allies"] = (target for target in player.relation_tags_many["allies"] if target is first)
+
+    assert set(player.relation_tags_many["allies"]) == {first}
+
+
+def test_relation_assignment_from_another_key() -> None:
+    world = tcod.ecs.Registry()
+    player = world["player"]
+    first, second = world["first"], world["second"]
+    player.relation_tags_many["allies"] = (first,)
+    player.relation_tags_many["rivals"] = (second,)
+
+    player.relation_tags_many["allies"] = player.relation_tags_many["rivals"]
+
+    assert set(player.relation_tags_many["allies"]) == {second}
+    assert set(player.relation_tags_many["rivals"]) == {second}
+
+
+def test_relation_set_operations() -> None:
+    world = tcod.ecs.Registry()
+    player = world["player"]
+    first, second, third = world["first"], world["second"], world["third"]
+    player.relation_tags_many["allies"] = (first, second)
+    allies = player.relation_tags_many["allies"]
+    other = {second, third}
+
+    assert allies | other == {first, second, third}
+    assert allies & other == {second}
+    assert allies - other == {first}
+    assert allies ^ other == {first, third}
+    assert set(allies) == {first, second}
+
+
+def test_tag_set_operations() -> None:
+    world = tcod.ecs.Registry()
+    player = world["player"]
+    player.tags |= {"visible", "alive"}
+    other = {"alive", "friendly"}
+    query = world.Q.all_of(tags=["visible"])
+    assert set(query) == {player}
+
+    assert player.tags | other == {"visible", "alive", "friendly"}
+    assert player.tags & other == {"alive"}
+    assert player.tags - other == {"visible"}
+    assert player.tags ^ other == {"visible", "friendly"}
+    assert set(player.tags) == {"visible", "alive"}
+    player.tags &= other
+    assert set(player.tags) == {"alive"}
+    assert set(query) == set()
+
+
 def test_relations() -> None:
     world = tcod.ecs.Registry()
     entity_a = world["A"]