Address review

- Use explicit joint type names
- Use `jnp.newaxis` instead of `None`
- Use dataclass for `JointType` instead of metaclass

Co-authored-by: Diego Ferigo <diego.ferigo@iit.it>

src/jaxsim/api/kin_dyn_parameters.py

@@ -394,8 +394,8 @@ def joint_transforms_and_motion_subspaces(
         # Extract the transforms and motion subspaces of the joints.
         # We stack the base transform W_H_B at index 0, and a dummy motion subspace
         # for either the fixed or free-floating joint connecting the world to the base.
-        pre_H_suc = jnp.vstack([W_H_B[None, ...], pre_H_suc_J])
-        S = jnp.vstack([jnp.zeros((6, 1))[None, ...], S_J])
+        pre_H_suc = jnp.vstack([W_H_B[jnp.newaxis, ...], pre_H_suc_J])
+        S = jnp.vstack([jnp.zeros((6, 1))[jnp.newaxis, ...], S_J])
 
         # Extract the successor-to-child fixed transforms.
         # Note that here we include also the index 0 since suc_H_child[0] stores the

src/jaxsim/math/joint_model.py

@@ -108,7 +108,7 @@ def build(description: ModelDescription) -> JointModel:
             # Static attributes
             joint_dofs=tuple([base_dofs] + [int(1) for _ in ordered_joints]),
             joint_names=tuple(["world_to_base"] + [j.name for j in ordered_joints]),
-            joint_types=tuple([JointType.F] + [j.jtype for j in ordered_joints]),
+            joint_types=tuple([JointType.Fixed] + [j.jtype for j in ordered_joints]),
             joint_axis=tuple([j.axis for j in ordered_joints]),
         )
 
@@ -265,9 +265,9 @@ def compute_P():
     pre_H_suc, S = jax.lax.switch(
         index=joint_type,
         branches=(
-            compute_F,  # JointType.F
-            compute_R,  # JointType.R
-            compute_P,  # JointType.P
+            compute_F,  # JointType.Fixed
+            compute_R,  # JointType.Revolute
+            compute_P,  # JointType.Prismatic
         ),
     )
 

src/jaxsim/parsers/descriptions/joint.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 import dataclasses
-from typing import Tuple, Union
+from typing import ClassVar, Tuple, Union
 
 import jax_dataclasses
 import numpy as np
@@ -13,31 +13,11 @@
 from .link import LinkDescription
 
 
-class _JointTypeMeta(type):
-    def __new__(cls, name, bases, dct):
-        cls_instance = super().__new__(cls, name, bases, dct)
-
-        # Assign integer values to the descriptors
-        cls_instance.F = 0
-        cls_instance.R = 1
-        cls_instance.P = 2
-
-        return cls_instance
-
-
-class JointType(metaclass=_JointTypeMeta):
-    """
-    Type of supported joints.
-    """
-
-    class F:
-        pass
-
-    class R:
-        pass
-
-    class P:
-        pass
+@dataclasses.dataclass(frozen=True)
+class JointType:
+    Fixed: ClassVar[int] = 0
+    Revolute: ClassVar[int] = 1
+    Prismatic: ClassVar[int] = 2
 
 
 @jax_dataclasses.pytree_dataclass

src/jaxsim/parsers/rod/parser.py

@@ -352,7 +352,7 @@ def build_model_description(
         considered_joints=[
             j.name
             for j in sdf_data.joint_descriptions
-            if j.jtype is not descriptions.JointType.F
+            if j.jtype is not descriptions.JointType.Fixed
         ],
     )
 

src/jaxsim/parsers/rod/utils.py

@@ -76,7 +76,7 @@ def joint_to_joint_type(
     joint_type = joint.type
 
     if joint_type == "fixed":
-        return descriptions.JointType.F
+        return descriptions.JointType.Fixed
 
     if not (axis.xyz is not None and axis.xyz.xyz is not None):
         raise ValueError("Failed to read axis xyz data")
@@ -86,10 +86,10 @@ def joint_to_joint_type(
     axis_xyz = axis_xyz / np.linalg.norm(axis_xyz)
 
     if joint_type in {"revolute", "continuous"}:
-        return descriptions.JointType.R
+        return descriptions.JointType.Revolute
 
     if joint_type == "prismatic":
-        return descriptions.JointType.P
+        return descriptions.JointType.Prismatic
 
     raise ValueError("Joint not supported", axis_xyz, joint_type)