Adding Tendon USD data classes and writers (isaac-sim#6265)

# Description

Adds the **tendons** schema-fragment API.

- `FixedTendonFragment` / `SpatialTendonFragment` markers +
`PhysxFixedTendonCfg` / `PhysxSpatialTendonCfg` in `isaaclab_physx`.
These are data carriers that override `func` to the existing
multi-instance `modify_fixed_tendon_properties` /
`modify_spatial_tendon_properties` writers (tune-not-apply), since
tendon attributes live under multi-instance `PhysxTendonAxisRootAPI:*`
schemas that the generic `apply_namespaced` writer cannot handle.
- The spawner `fixed_tendons_props` / `spatial_tendons_props` slots now
also accept fragment lists.

This PR is purely **additive** and self-contained: it builds only on the
single-namespace schema-fragment base (`SchemaFragment` +
`apply_namespaced`) already in `develop`, existing call sites are
untouched (a transition bridge routes legacy single cfgs to the existing
`define_`/`modify_` writers), and it does **not** depend on any other
open PR.

## Type of change

- New feature (non-breaking change which adds functionality)

## Screenshots

N/A — non-visual API change.

## Checklist

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have added a changelog fragment under
`source/<pkg>/changelog.d/` for every touched package (do **not** edit
`CHANGELOG.rst` or bump `extension.toml` — CI handles that)
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

Author: vidurv-nvidia

source/isaaclab/changelog.d/vidurv-schema-frag-tendons.minor.rst

@@ -0,0 +1,29 @@
+Added
+^^^^^
+
+* Added the tendon schema-fragment markers
+  :class:`~isaaclab.sim.schemas.FixedTendonFragment` and
+  :class:`~isaaclab.sim.schemas.SpatialTendonFragment`, which type the spawner
+  ``fixed_tendons_props`` / ``spatial_tendons_props`` slots.
+* Added :func:`~isaaclab.sim.schemas.apply_fixed_tendon_properties` and
+  :func:`~isaaclab.sim.schemas.apply_spatial_tendon_properties`, the family writers that
+  dispatch a list of tendon fragments via each fragment's ``func``. Tendons are a
+  *tune-not-apply* family, so the writers tune the existing multi-instance tendon schemas
+  without applying a new anchor schema.
+
+Changed
+^^^^^^^
+
+* Changed the spawner ``fixed_tendons_props`` / ``spatial_tendons_props`` slots
+  (:attr:`~isaaclab.sim.spawners.from_files.FileCfg.fixed_tendons_props`,
+  :attr:`~isaaclab.sim.spawners.from_files.FileCfg.spatial_tendons_props`) to also accept one
+  or more tendon fragments. Legacy single cfgs continue to work through a transition bridge in
+  the spawn writer.
+* :func:`~isaaclab.sim.schemas.apply_fixed_tendon_properties` and
+  :func:`~isaaclab.sim.schemas.apply_spatial_tendon_properties` now raise
+  ``ValueError`` when the prim at ``prim_path`` does not exist in the stage.
+  Callers that previously relied on an implicit no-op for invalid paths must
+  either validate the path beforehand or catch ``ValueError``.
+  The aggregated return value is now ``False`` whenever any fragment applier
+  reports failure; callers must not assume the return is always ``True`` even
+  when the prim is valid.

source/isaaclab/isaaclab/sim/__init__.pyi

@@ -44,6 +44,7 @@ __all__ = [
     "ConvexHullPropertiesCfg",
     "DeformableBodyPropertiesBaseCfg",
     "DeformableBodyPropertiesCfg",
+    "FixedTendonFragment",
     "FixedTendonPropertiesCfg",
     "JointDriveBaseCfg",
     "MassCfg",
@@ -64,12 +65,15 @@ __all__ = [
     "RigidBodyBaseCfg",
     "RigidBodyFragment",
     "SchemaFragment",
+    "SpatialTendonFragment",
     "UsdPhysicsCollisionCfg",
     "UsdPhysicsRigidBodyCfg",
     "apply_collision_properties",
+    "apply_fixed_tendon_properties",
     "apply_mass_properties",
     "apply_namespaced",
     "apply_rigid_body_properties",
+    "apply_spatial_tendon_properties",
     "SDFMeshPropertiesCfg",
     "SpatialTendonPropertiesCfg",
     "TriangleMeshPropertiesCfg",
@@ -226,6 +230,7 @@ from .schemas import (
     ConvexHullPropertiesCfg,
     DeformableBodyPropertiesBaseCfg,
     DeformableBodyPropertiesCfg,
+    FixedTendonFragment,
     FixedTendonPropertiesCfg,
     JointDriveBaseCfg,
     MassCfg,
@@ -238,16 +243,19 @@ from .schemas import (
     RigidBodyFragment,
     SchemaFragment,
     SDFMeshPropertiesCfg,
+    SpatialTendonFragment,
     SpatialTendonPropertiesCfg,
     TriangleMeshPropertiesCfg,
     TriangleMeshSimplificationPropertiesCfg,
     UsdPhysicsCollisionCfg,
     UsdPhysicsRigidBodyCfg,
     activate_contact_sensors,
     apply_collision_properties,
+    apply_fixed_tendon_properties,
     apply_mass_properties,
     apply_namespaced,
     apply_rigid_body_properties,
+    apply_spatial_tendon_properties,
     define_articulation_root_properties,
     define_collision_properties,
     define_deformable_body_properties,

source/isaaclab/isaaclab/sim/schemas/__init__.pyi

@@ -9,9 +9,11 @@ __all__ = [
     "USD_MESH_COLLISION_CFGS",
     "activate_contact_sensors",
     "apply_collision_properties",
+    "apply_fixed_tendon_properties",
     "apply_mass_properties",
     "apply_namespaced",
     "apply_rigid_body_properties",
+    "apply_spatial_tendon_properties",
     "define_actuator_properties",
     "define_articulation_root_properties",
     "define_collision_properties",
@@ -35,13 +37,15 @@ __all__ = [
     "CollisionFragment",
     "DeformableBodyPropertiesBaseCfg",
     "DeformableBodyPropertiesCfg",
+    "FixedTendonFragment",
     "JointDriveBaseCfg",
     "MassCfg",
     "MassFragment",
     "MassPropertiesCfg",
     "MeshCollisionBaseCfg",
     "RigidBodyFragment",
     "SchemaFragment",
+    "SpatialTendonFragment",
     "UsdPhysicsCollisionCfg",
     "UsdPhysicsRigidBodyCfg",
     "MujocoJointDrivePropertiesCfg",
@@ -62,9 +66,11 @@ from .schemas import (
     USD_MESH_COLLISION_CFGS,
     activate_contact_sensors,
     apply_collision_properties,
+    apply_fixed_tendon_properties,
     apply_mass_properties,
     apply_namespaced,
     apply_rigid_body_properties,
+    apply_spatial_tendon_properties,
     define_articulation_root_properties,
     define_collision_properties,
     define_deformable_body_properties,
@@ -92,6 +98,7 @@ from .schemas_cfg import (
     CollisionFragment,
     DeformableBodyPropertiesBaseCfg,
     DeformableBodyPropertiesCfg,
+    FixedTendonFragment,
     JointDriveBaseCfg,
     MassCfg,
     MassFragment,
@@ -100,6 +107,7 @@ from .schemas_cfg import (
     RigidBodyBaseCfg,
     RigidBodyFragment,
     SchemaFragment,
+    SpatialTendonFragment,
     UsdPhysicsCollisionCfg,
     UsdPhysicsRigidBodyCfg,
 )

source/isaaclab/isaaclab/sim/schemas/schemas.py

@@ -990,6 +990,48 @@ def modify_joint_drive_properties(
 """
 
 
+def apply_fixed_tendon_properties(
+    prim_path: str, fragments: Iterable[schemas_cfg.FixedTendonFragment], stage: Usd.Stage | None = None
+) -> bool:
+    """Apply a list of fixed-tendon fragments to a prim.
+
+    Fixed tendons are a *tune-not-apply* family: the applied ``PhysxTendonAxisRootAPI``
+    multi-instance schemas already exist on the prim (authored in the source asset). This writer
+    therefore applies no anchor schema; it only dispatches each fragment via its
+    :attr:`~isaaclab.sim.schemas.SchemaFragment.func`, which tunes the existing instances.
+    Backend fragments carry backend-specific funcs, so core never imports a backend.
+
+    Each fragment tunes only its own schema and returns ``False`` when that schema is not
+    present on the prim. A prim carries a single tendon backend, so compose backends across
+    prims rather than mixing PhysX and Mujoco fragments in one list on one prim.
+
+    Args:
+        prim_path: The prim path to apply the fixed-tendon schemas on.
+        fragments: An iterable of :class:`~isaaclab.sim.schemas.FixedTendonFragment` instances.
+        stage: The stage where to find the prim. Defaults to None, in which case the current
+            stage is used.
+
+    Returns:
+        True if all fragments applied successfully, False if any fragment reported failure.
+
+    Raises:
+        ValueError: If the prim at ``prim_path`` is not valid.
+    """
+    if stage is None:
+        stage = get_current_stage()
+    prim = stage.GetPrimAtPath(prim_path)
+    # fail loudly on an invalid path (matches the sibling apply_* writers)
+    if not prim.IsValid():
+        raise ValueError(f"Prim path '{prim_path}' is not valid.")
+    # tune-not-apply: the PhysxTendonAxisRootAPI instances already exist; apply no anchor.
+    # aggregate per-fragment results so a reported failure is not silently masked.
+    success = True
+    for cfg in fragments:
+        func = cfg.func if callable(cfg.func) else string_to_callable(cfg.func)
+        success = bool(func(cfg, prim_path, stage)) and success
+    return success
+
+
 @apply_nested
 def modify_fixed_tendon_properties(
     prim_path: str, cfg: schemas_cfg.PhysxFixedTendonPropertiesCfg, stage: Usd.Stage | None = None
@@ -1021,6 +1063,9 @@ def modify_fixed_tendon_properties(
     Raises:
         ValueError: If the input prim path is not valid.
     """
+    # Retained for backward compatibility with callers passing PhysxFixedTendonPropertiesCfg
+    # directly. Will be removed in a future release once callers adopt the fragment-based
+    # apply_fixed_tendon_properties path.
     # get stage handle
     if stage is None:
         stage = get_current_stage()
@@ -1048,6 +1093,7 @@ def modify_fixed_tendon_properties(
                     camel_case=False,
                 )
     else:
+        # NOTE: ``mjc:*`` branch (``MjcTendon`` prim) kept inline; future split candidate into isaaclab_newton.
         # only stiffness and damping in the cfg map to mjc attributes
         for attr_name, value in cfg.items():
             safe_set_attribute_on_usd_prim(
@@ -1062,6 +1108,50 @@ def modify_fixed_tendon_properties(
 """
 
 
+def apply_spatial_tendon_properties(
+    prim_path: str, fragments: Iterable[schemas_cfg.SpatialTendonFragment], stage: Usd.Stage | None = None
+) -> bool:
+    """Apply a list of spatial-tendon fragments to a prim.
+
+    Spatial tendons are a *tune-not-apply* family: the applied
+    ``PhysxTendonAttachmentRootAPI`` / ``PhysxTendonAttachmentLeafAPI`` multi-instance schemas
+    already exist on the prim (authored in the source asset). This writer therefore applies no
+    anchor schema; it only dispatches each fragment via its
+    :attr:`~isaaclab.sim.schemas.SchemaFragment.func`, which tunes the existing instances.
+    Backend fragments carry backend-specific funcs, so core never imports a backend.
+
+    Each fragment tunes only its own schema and returns ``False`` when that schema is not
+    present on the prim. A prim carries a single tendon backend, so compose backends across
+    prims rather than mixing PhysX and Mujoco fragments in one list on one prim.
+
+    Args:
+        prim_path: The prim path to apply the spatial-tendon schemas on.
+        fragments: An iterable of :class:`~isaaclab.sim.schemas.SpatialTendonFragment` instances.
+        stage: The stage where to find the prim. Defaults to None, in which case the current
+            stage is used.
+
+    Returns:
+        True if all fragments applied successfully, False if any fragment reported failure.
+
+    Raises:
+        ValueError: If the prim at ``prim_path`` is not valid.
+    """
+    if stage is None:
+        stage = get_current_stage()
+    prim = stage.GetPrimAtPath(prim_path)
+    # fail loudly on an invalid path (matches the sibling apply_* writers)
+    if not prim.IsValid():
+        raise ValueError(f"Prim path '{prim_path}' is not valid.")
+    # tune-not-apply: the PhysxTendonAttachmentRootAPI / PhysxTendonAttachmentLeafAPI instances
+    # already exist; apply no anchor.
+    # aggregate per-fragment results so a reported failure is not silently masked.
+    success = True
+    for cfg in fragments:
+        func = cfg.func if callable(cfg.func) else string_to_callable(cfg.func)
+        success = bool(func(cfg, prim_path, stage)) and success
+    return success
+
+
 @apply_nested
 def modify_spatial_tendon_properties(
     prim_path: str, cfg: schemas_cfg.PhysxSpatialTendonPropertiesCfg, stage: Usd.Stage | None = None
@@ -1095,6 +1185,9 @@ def modify_spatial_tendon_properties(
     Raises:
         ValueError: If the input prim path is not valid.
     """
+    # Retained for backward compatibility with callers passing PhysxSpatialTendonPropertiesCfg
+    # directly. Will be removed in a future release once callers adopt the fragment-based
+    # apply_spatial_tendon_properties path.
     # obtain stage
     if stage is None:
         stage = get_current_stage()

source/isaaclab/isaaclab/sim/schemas/schemas_cfg.py

@@ -183,6 +183,35 @@ class CollisionFragment(SchemaFragment):
     pass
 
 
+@configclass
+class FixedTendonFragment(SchemaFragment):
+    """Marker base for fixed-tendon fragments; types the ``fixed_tendons_props`` slot.
+
+    Fixed tendons are a *tune-not-apply* family: the applied ``PhysxTendonAxisRootAPI``
+    multi-instance schemas already exist on the prim (authored in the source asset), so the
+    family writer (:func:`~isaaclab.sim.schemas.apply_fixed_tendon_properties`) does not apply
+    any anchor schema; it only tunes the existing instances via each fragment's
+    :attr:`~isaaclab.sim.schemas.SchemaFragment.func`.
+    """
+
+    pass
+
+
+@configclass
+class SpatialTendonFragment(SchemaFragment):
+    """Marker base for spatial-tendon fragments; types the ``spatial_tendons_props`` slot.
+
+    Spatial tendons are a *tune-not-apply* family: the applied
+    ``PhysxTendonAttachmentRootAPI`` / ``PhysxTendonAttachmentLeafAPI`` multi-instance schemas
+    already exist on the prim (authored in the source asset), so the family writer
+    (:func:`~isaaclab.sim.schemas.apply_spatial_tendon_properties`) does not apply any anchor
+    schema; it only tunes the existing instances via each fragment's
+    :attr:`~isaaclab.sim.schemas.SchemaFragment.func`.
+    """
+
+    pass
+
+
 @configclass
 class UsdPhysicsCollisionCfg(CollisionFragment):
     """``physics:*`` collision attributes from `UsdPhysics.CollisionAPI`_.

source/isaaclab/isaaclab/sim/spawners/from_files/from_files.py

@@ -372,9 +372,33 @@ def _spawn_from_usd_file(
         schemas.modify_articulation_root_properties(prim_path, cfg.articulation_props)
     # modify tendon properties
     if cfg.fixed_tendons_props is not None:
-        schemas.modify_fixed_tendon_properties(prim_path, cfg.fixed_tendons_props)
+        # transition shim, remove later: fragment(s) -> apply_*; legacy cfg -> modify_*
+        # normalize a single fragment to a list so the convenience form (and an empty list) route like a list
+        fixed_tendon_frags = (
+            [cfg.fixed_tendons_props]
+            if isinstance(cfg.fixed_tendons_props, schemas.SchemaFragment)
+            else cfg.fixed_tendons_props
+        )
+        if isinstance(fixed_tendon_frags, (list, tuple)) and all(
+            isinstance(f, schemas.SchemaFragment) for f in fixed_tendon_frags
+        ):
+            schemas.apply_fixed_tendon_properties(prim_path, fixed_tendon_frags)
+        else:
+            schemas.modify_fixed_tendon_properties(prim_path, cfg.fixed_tendons_props)
     if cfg.spatial_tendons_props is not None:
-        schemas.modify_spatial_tendon_properties(prim_path, cfg.spatial_tendons_props)
+        # transition shim, remove later: fragment(s) -> apply_*; legacy cfg -> modify_*
+        # normalize a single fragment to a list so the convenience form (and an empty list) route like a list
+        spatial_tendon_frags = (
+            [cfg.spatial_tendons_props]
+            if isinstance(cfg.spatial_tendons_props, schemas.SchemaFragment)
+            else cfg.spatial_tendons_props
+        )
+        if isinstance(spatial_tendon_frags, (list, tuple)) and all(
+            isinstance(f, schemas.SchemaFragment) for f in spatial_tendon_frags
+        ):
+            schemas.apply_spatial_tendon_properties(prim_path, spatial_tendon_frags)
+        else:
+            schemas.modify_spatial_tendon_properties(prim_path, cfg.spatial_tendons_props)
     # define drive API on the joints
     # note: these are only for setting low-level simulation properties. all others should be set or are
     #  and overridden by the articulation/actuator properties.

source/isaaclab/isaaclab/sim/spawners/from_files/from_files_cfg.py

@@ -37,11 +37,23 @@ class FileCfg(RigidObjectSpawnerCfg, DeformableObjectSpawnerCfg):
     articulation_props: schemas.ArticulationRootPropertiesCfg | None = None
     """Properties to apply to the articulation root."""
 
-    fixed_tendons_props: schemas.FixedTendonPropertiesCfg | None = None
-    """Properties to apply to the fixed tendons (if any)."""
+    fixed_tendons_props: (
+        schemas.FixedTendonPropertiesCfg | schemas.FixedTendonFragment | list[schemas.FixedTendonFragment] | None
+    ) = None
+    """Properties to apply to the fixed tendons (if any).
 
-    spatial_tendons_props: schemas.SpatialTendonPropertiesCfg | None = None
-    """Properties to apply to the spatial tendons (if any)."""
+    Accepts either the legacy :class:`~isaaclab_physx.sim.schemas.PhysxFixedTendonPropertiesCfg`
+    or one or more :class:`~isaaclab.sim.schemas.FixedTendonFragment` instances.
+    """
+
+    spatial_tendons_props: (
+        schemas.SpatialTendonPropertiesCfg | schemas.SpatialTendonFragment | list[schemas.SpatialTendonFragment] | None
+    ) = None
+    """Properties to apply to the spatial tendons (if any).
+
+    Accepts either the legacy :class:`~isaaclab_physx.sim.schemas.PhysxSpatialTendonPropertiesCfg`
+    or one or more :class:`~isaaclab.sim.schemas.SpatialTendonFragment` instances.
+    """
 
     joint_drive_props: schemas.JointDriveBaseCfg | None = None
     """Properties to apply to a joint.