nit: account for empty slots

marimo/_plugins/core/json_encoder.py

@@ -146,17 +146,23 @@ def _convert_to_json(self, o: Any) -> Any:
             }
 
         # Handle objects with __slots__
-        if getattr(o, "__slots__", None):
-            # Reported error that sometimes poorly formed objects do get passed
-            # in.
-            assert isinstance(o.__slots__, (tuple, list)), (
-                f"Unexpected __slots__ type for {o.__class__.__name__}",
-            )
-            return {
-                slot: self._convert_to_json(getattr(o, slot))
-                for slot in o.__slots__  # type: ignore
-                if hasattr(o, slot)
-            }
+        if hasattr(o, "__slots__"):
+            slots = getattr(o, "__slots__", None)
+            if slots is not None:
+                # Reported error that sometimes poorly formed objects do get passed
+                # in.
+                try:
+                    slots = iter(slots)
+                except TypeError as e:
+                    raise TypeError(
+                        "__slots__ expected to be tuple or list (or at least "
+                        f"iterable), but got {type(slots)} for {type(o)}"
+                    ) from e
+                return {
+                    slot: self._convert_to_json(getattr(o, slot))
+                    for slot in slots
+                    if hasattr(o, slot)
+                }
 
         # Handle custom objects with __dict__
         if hasattr(o, "__dict__"):

tests/_plugins/core/test_json_encoder.py

@@ -438,3 +438,18 @@ class InvalidClass: ...
 
     encoded = json.dumps(invalid_obj, cls=WebComponentEncoder)
     assert encoded == '{"__slots__": null}'
+
+
+def test_empty_slots() -> None:
+    class ExClass:
+        __slots__ = []
+
+        # With a property (as sanity check)
+        @property
+        def one(self):
+            return 1
+
+    obj = ExClass()
+
+    encoded = json.dumps(obj, cls=WebComponentEncoder)
+    assert encoded == "{}"