fix(models): make KinovaGen3 load via robot_descriptions, add XACRO_ARGS support

src/roboticstoolbox/models/URDF/KinovaGen3.py

@@ -31,7 +31,7 @@ class KinovaGen3(URDFRobot):
 
     def __init__(self):
         super().__init__(
-            "kortex_description/robots/gen3.xacro",
+            "gen3",
             manufacturer="Kinova",
             gripper_link_index=10,
         )

src/roboticstoolbox/models/URDF/URDFRobot.py

@@ -116,7 +116,7 @@ def _load_rd_module(robot_name: str):
     ) from last_error
 
 
-def _load_urdf_from_RD(robot_name: str) -> Path:
+def _load_urdf_from_RD(robot_name: str) -> "tuple[Path, dict | None]":
     """Fetch the URDF/xacro path from robot_descriptions and register its packages.
 
     robot_descriptions models expose different path attributes depending on their
@@ -125,6 +125,13 @@ def _load_urdf_from_RD(robot_name: str) -> Path:
       - XACRO_PATH — a xacro source that must be processed by xacrodoc
                      (e.g. j2n6s200, ur5, kinova family)
     Both are passed to XacroDoc.from_file(), which handles either format.
+
+    Some xacro-based models also expose ``XACRO_ARGS`` — substitution
+    values required to compile the file at all (e.g. Kinova Gen3's
+    ``{"dof": "7"}``, which selects the 6dof/7dof arm variant; several UR
+    and xArm variants have the same pattern). Returned alongside the path
+    so the caller can forward them to xacrodoc as ``subargs`` — without
+    them, xacro fails on an unresolved ``$(arg ...)``/property reference.
     """
     module = _load_rd_module(robot_name)
 
@@ -138,7 +145,7 @@ def _load_urdf_from_RD(robot_name: str) -> Path:
             "URDF_PATH nor XACRO_PATH."
         )
     _register_rd_packages(urdf_path)
-    return urdf_path
+    return urdf_path, getattr(module, "XACRO_ARGS", None)
 
 
 def _parse_urdf(urdf_str: str):
@@ -285,10 +292,11 @@ def URDF_file(
     pkg_map = {d.name: str(d) for d in xacro_root.iterdir() if d.is_dir()}
     packages.update_package_cache(pkg_map)
 
+    xacro_args = None
     if isinstance(file, str):
         file = Path(file)
         if file.suffix not in (".urdf", ".xacro"):
-            file = Path(_load_urdf_from_RD(str(file)))
+            file, xacro_args = _load_urdf_from_RD(str(file))
 
     resolved_path = None
     if isinstance(file, Path):
@@ -299,9 +307,11 @@ def URDF_file(
             # mirrors XacroDoc.from_file()'s own package-discovery step,
             # since we bypass from_file() here to patch the text first
             packages.walk_up_from(file)
-            doc = XacroDoc.from_string(patch(file.read_text()), rootdir=file.parent)
+            doc = XacroDoc.from_string(
+                patch(file.read_text()), rootdir=file.parent, subargs=xacro_args
+            )
         else:
-            doc = XacroDoc.from_file(file)
+            doc = XacroDoc.from_file(file, subargs=xacro_args)
     else:
         text = file.read()
         if patch is not None: