fix(serializer): traverse full MRO when serialising objects with __slots__

langfuse/_utils/serializer.py

@@ -151,9 +151,18 @@ def _default_inner(self, obj: Any) -> Any:
                 return [self.default(item) for item in obj]
 
             if hasattr(obj, "__slots__"):
+                # Collect slots from the entire MRO so that attributes defined
+                # on parent classes are not silently dropped.  Accessing
+                # obj.__slots__ only returns the slots of the *immediate* class;
+                # inherited slots live on their respective base classes.
+                all_slots = [
+                    slot
+                    for cls in type(obj).__mro__
+                    for slot in getattr(cls, "__slots__", ())
+                ]
                 return {
                     slot: self.default(getattr(obj, slot, None))
-                    for slot in obj.__slots__
+                    for slot in all_slots
                 }
             elif hasattr(obj, "__dict__"):
                 obj_id = id(obj)

tests/unit/test_serializer.py

@@ -298,3 +298,70 @@ def test_dict_with_non_string_keys_is_serialized(input_obj, expected):
     result = json.loads(EventSerializer().encode(input_obj))
 
     assert result == expected
+
+
+# ---------------------------------------------------------------------------
+# Regression tests: __slots__ MRO traversal
+#
+# EventSerializer previously only iterated obj.__slots__, which returns the
+# slots of the immediate class only.  Attributes defined on parent classes via
+# __slots__ were silently dropped.
+# ---------------------------------------------------------------------------
+
+
+class _SlotBase:
+    __slots__ = ("base_attr",)
+
+    def __init__(self):
+        self.base_attr = "from_base"
+
+
+class _SlotChild(_SlotBase):
+    __slots__ = ("child_attr",)
+
+    def __init__(self):
+        super().__init__()
+        self.child_attr = "from_child"
+
+
+class _SlotGrandchild(_SlotChild):
+    __slots__ = ("grandchild_attr",)
+
+    def __init__(self):
+        super().__init__()
+        self.grandchild_attr = "from_grandchild"
+
+
+def test_slots_immediate_class_serialised():
+    """Child-class slots must appear in the serialised output."""
+    serializer = EventSerializer()
+    obj = _SlotChild()
+    result = json.loads(serializer.encode(obj))
+    assert result.get("child_attr") == "from_child", (
+        f"child_attr must be serialised; got: {result}"
+    )
+
+
+def test_slots_inherited_from_parent_serialised():
+    """Parent-class slots must not be silently dropped during serialisation.
+
+    Before the fix, obj.__slots__ returned only ('child_attr',), so
+    base_attr was completely absent from the output.
+    """
+    serializer = EventSerializer()
+    obj = _SlotChild()
+    result = json.loads(serializer.encode(obj))
+    assert result.get("base_attr") == "from_base", (
+        "base_attr is defined in the parent class's __slots__ and must appear "
+        f"in the serialised output; got: {result}"
+    )
+
+
+def test_slots_deep_inheritance_chain():
+    """All slots across a 3-level MRO must be serialised."""
+    serializer = EventSerializer()
+    obj = _SlotGrandchild()
+    result = json.loads(serializer.encode(obj))
+    assert result.get("base_attr") == "from_base", f"base_attr missing from: {result}"
+    assert result.get("child_attr") == "from_child", f"child_attr missing from: {result}"
+    assert result.get("grandchild_attr") == "from_grandchild", f"grandchild_attr missing from: {result}"