Support IAccessible2 labelled-by relation

source/IAccessibleHandler/types.py

@@ -29,3 +29,4 @@ class RelationType(str, enum.Enum):
 	CONTROLLER_FOR = "controllerFor"
 	ERROR = "error"
 	ERROR_FOR = "errorFor"
+	LABELLED_BY = "labelledBy"

source/NVDAObjects/IAccessible/__init__.py

@@ -3,13 +3,13 @@
 # This file is covered by the GNU General Public License.
 # See the file COPYING for more details.
 
+from __future__ import annotations
 import typing
 from typing import (
 	Generator,
 	Optional,
 	Tuple,
 	Union,
-	List,
 )
 
 from comtypes.automation import IEnumVARIANT, VARIANT
@@ -1162,13 +1162,20 @@ def isPointInObject(self, x, y):
 			return False
 		return True
 
-	def _get_labeledBy(self):
+	def _get_labeledBy(self) -> IAccessible | None:
+		label = self._getIA2RelationFirstTarget(IAccessibleHandler.RelationType.LABELLED_BY)
+		if label:
+			return label
+
 		try:
-			(pacc, accChild) = IAccessibleHandler.accNavigate(
+			ret = IAccessibleHandler.accNavigate(
 				self.IAccessibleObject,
 				self.IAccessibleChildID,
 				IAccessibleHandler.NAVRELATION_LABELLED_BY,
 			)
+			if not ret:
+				return None
+			(pacc, accChild) = ret
 			obj = IAccessible(IAccessibleObject=pacc, IAccessibleChildID=accChild)
 			return obj
 		except COMError:
@@ -1940,7 +1947,7 @@ def _get_detailsRelations(self) -> Tuple["IAccessible"]:
 		# due to caching of baseObject.AutoPropertyObject, do not attempt to return a generator.
 		return tuple(detailsRelsGen)
 
-	def _get_controllerFor(self) -> List[NVDAObject]:
+	def _get_controllerFor(self) -> typing.List[NVDAObject]:
 		control = self._getIA2RelationFirstTarget(IAccessibleHandler.RelationType.CONTROLLER_FOR)
 		if control:
 			return [control]

user_docs/en/changes.md

@@ -99,6 +99,7 @@ Add-ons will need to be re-tested and have their manifest updated.
 * Removed the requirement to indent function parameter lists by two tabs from NVDA's Coding Standards, to be compatible with modern automatic linting. (#17126, @XLTechie)
 * Added the [VS Code workspace configuration for NVDA](https://nvaccess.org/nvaccess/vscode-nvda) as a git submodule. (#17003)
 * In the `brailleTables` module, a `getDefaultTableForCurrentLang` function has been added (#17222, @nvdaes)
+* Retrieving the "labeledBy" property now works for objects in applications implementing the "labelled-by" IAccessible2 relation and no longer triggers an error. (#17436, @michaelweghorn)
 
 #### API Breaking Changes