Python: fix kernel_function decorator mutating shared Annotated metadata dicts

python/semantic_kernel/functions/kernel_function_decorator.py

@@ -71,7 +71,9 @@ def decorator(func: Callable[..., object]) -> Callable[..., object]:
         setattr(func, "__kernel_function_parameters__", annotations)
 
         return_annotation = (
-            _parse_parameter("return", func_sig.return_annotation, None) if func_sig.return_annotation else {}
+            _parse_parameter("return", func_sig.return_annotation, Parameter.empty)
+            if func_sig.return_annotation
+            else {}
         )
         setattr(func, "__kernel_function_return_type__", return_annotation.get("type_", "None"))
         setattr(func, "__kernel_function_return_type_object__", return_annotation.get("type_object", None))
@@ -120,7 +122,8 @@ def _process_signature(func_sig: Signature) -> list[dict[str, Any]]:
         if arg.name == "self":
             continue
         annotation = arg.annotation
-        default = arg.default if arg.default != arg.empty else None
+        # Parameter.empty means "no default"; a default of None is still a default.
+        default = arg.default
         parsed_annotation = _parse_parameter(arg.name, annotation, default)
         if get_origin(annotation) is Annotated or get_origin(annotation) in {Union, types.UnionType}:
             underlying_type = _get_underlying_type(annotation)
@@ -137,7 +140,7 @@ def _parse_parameter(name: str, param: Any, default: Any) -> dict[str, Any]:
     logger.debug(f"Parsing param: {name}")
     logger.debug(f"Parsing annotation: {param}")
     ret: dict[str, Any] = {"name": name}
-    if default is not None:
+    if default is not Parameter.empty:
         ret["default_value"] = default
         ret["is_required"] = False
     else:
@@ -151,13 +154,14 @@ def _parse_parameter(name: str, param: Any, default: Any) -> dict[str, Any]:
                 if isinstance(meta, str):
                     ret["description"] = meta
                 elif isinstance(meta, dict):
+                    meta = meta.copy()
                     # only override from the metadata if it is not already set
                     if "description" not in ret and (description := meta.pop("description", None)):
                         ret["description"] = description
                     ret.update(meta)
                 else:
                     logger.debug(f"Unknown metadata type: {meta}")
-        if hasattr(param, "__origin__"):
+        if hasattr(param, "__origin__") and param.__origin__ not in (Union, types.UnionType):
             ret.update(_parse_parameter(name, param.__origin__, default))
         if hasattr(param, "__args__"):
             args = []

python/tests/unit/functions/test_kernel_function_decorators.py

@@ -280,3 +280,37 @@ def test_annotation_parsing(name, annotation, description, type_, is_required):
     assert description == annotation_dict.get("description")
     assert type_ == annotation_dict["type_"]
     assert is_required == annotation_dict["is_required"]
+
+
+def test_annotation_description_precedence_across_metadata_entries():
+    """A later dict metadata entry overrides an earlier string description."""
+    meta = {"description": "later dictionary"}
+
+    @kernel_function
+    def func(arg: Annotated[int, "earlier string", meta]) -> str:
+        return "ok"
+
+    assert func.__kernel_function_parameters__[0]["description"] == "later dictionary"
+    # The caller-owned dict is untouched by decoration.
+    assert meta == {"description": "later dictionary"}
+
+
+def test_shared_annotation_metadata_dict_not_mutated():
+    """Decorating with one shared Annotated metadata dict keeps it usable twice."""
+    shared_meta = {"description": "shared description"}
+
+    @kernel_function
+    def func_one(arg: Annotated[int, shared_meta]) -> str:
+        return "one"
+
+    @kernel_function
+    def func_two(arg: Annotated[int, shared_meta]) -> str:
+        return "two"
+
+    assert shared_meta == {"description": "shared description"}
+    descriptions = [
+        param["description"]
+        for func in (func_one, func_two)
+        for param in func.__kernel_function_parameters__
+    ]
+    assert descriptions == ["shared description", "shared description"]