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newton-api-design

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Use when designing, adding, or reviewing public API for the Newton physics engine — class names, method signatures, type hints, docstrings, or parameter conventions. Also use when unsure if new API conforms to project conventions.

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Newton API Design Conventions

Detailed patterns that supplement AGENTS.md. Read AGENTS.md first for the basics (prefix-first naming, PEP 604, Google-style docstrings, SI units, Sphinx cross-refs).

Builder Method Signature Template

All ModelBuilder.add_shape_* methods follow this parameter order:

def add_shape_cone(
    self,
    body: int,
    xform: Transform | None = None,
    # shape-specific params here (radius, half_height, etc.)
    radius: float = 1.0,
    half_height: float = 0.5,
    cfg: ShapeConfig | None = None,
    as_site: bool = False,
    color: Vec3 | None = None,
    label: str | None = None,
    custom_attributes: dict[str, Any] | None = None,
) -> int:
    """Adds a cone collision shape to a body.

    Args:
        body: Index of the parent body. Use -1 for static shapes.
        xform: Transform in parent body's local frame. If ``None``,
            identity transform is used.
        radius: Cone base radius [m].
        half_height: Half the cone height [m].
        cfg: Shape configuration. If ``None``, uses
            :attr:`default_shape_cfg`.
        as_site: If ``True``, creates a site instead of a collision shape.
        color: Optional display RGB color in [0, 1]. If ``None``, uses
            the per-shape palette color.
        label: Optional label for identifying the shape.
        custom_attributes: Dictionary of custom attribute names to values.

    Returns:
        Index of the newly added shape.
    """

Key conventions:

  • xform (not tf, transform, or pose) — always Transform | None = None
  • cfg (not config, shape_config) — always ShapeConfig | None = None
  • body, color, label, custom_attributes — standard params on all builder methods
  • Defaults are None, not constructed objects like wp.transform()

Nested Classes

Use IntEnum (not Enum with strings) for enumerations:

class Model:
    class AttributeAssignment(IntEnum):
        MODEL = 0
        STATE = 1

When an IntEnum includes a NONE member, define it first at 0:

class GeoType(IntEnum):
    NONE = 0
    PLANE = 1
    HFIELD = 2

This keeps the sentinel value stable and leaves room to append future real members at the end instead of inserting them before a trailing NONE.

Dataclass field docstrings go on the line immediately below the field:

@dataclass
class ShapeConfig:
    density: float = 1000.0
    """The density of the shape material."""
    ke: float = 2.5e3
    """The contact elastic stiffness."""

Array Documentation Format

Annotate Warp arrays with the dtype, e.g. wp.array[wp.vec3], wp.array2d[float], wp.array[wp.spatial_vector] | None. Document units and shape in the docstring.

"""Rigid body velocities [m/s, rad/s], shape [body_count]."""
"""Joint forces [N or N·m], shape [joint_dof_count]."""
"""Contact points [m], shape [count, 3]."""

For compound arrays, list per-component units:

"""[0] k_mu [Pa], [1] k_lambda [Pa], ..."""

Use wp.array[X] for 1-D, wp.array2d[X] for 2-D, and wp.array[Any] for polymorphic dtypes.

Quick Checklist

When reviewing new API, verify:

  • Parameters use project vocabulary (xform, cfg, body, label)
  • Defaults are None, not constructed objects
  • Nested enumerations use IntEnum with int values
  • Enumerations with NONE define NONE = 0 first
  • Dataclass fields have docstrings on the line below
  • Warp array annotations include the dtype (e.g. wp.array[wp.vec3]); docstrings give units and shape
  • Builder methods include as_site, color, label, custom_attributes