Add unit-aware Param() field and with_units()/to_example() to meshly

python/README.md

@@ -6,6 +6,9 @@ A Python library for efficient 3D mesh serialization using [meshoptimizer](https
 
 ```bash
 pip install meshly
+
+# With unit conversion support (pint)
+pip install meshly[units]
 ```
 
 ## Features
@@ -15,6 +18,7 @@ pip install meshly
 - **`Packable`**: Base class for automatic numpy/JAX array serialization to zip files
 - **`Mesh`**: 3D mesh representation extending Packable with meshoptimizer encoding. Use factory methods: `from_triangles()`, `from_polygons()`, `create()`
 - **`ArrayUtils`**: Utility class for extracting/reconstructing individual arrays
+- **`Param`**: Unit-aware parameter field for Pydantic models (drop-in replacement for `Field`)
 - **`PackableStore`**: File-based store for persistent storage with deduplication
 - **`LazyModel`**: Lazy proxy that defers asset loading until field access
 - **`Resource`**: Binary data reference that serializes by content checksum
@@ -28,6 +32,13 @@ pip install meshly
 - **`IndexSequence`**: Optimized encoding for mesh indices (1D array)
 - **`InlineArray`**: Array serialized as inline JSON list (no binary compression)
 
+### Unit-Aware Parameters
+
+- **`Param()`**: Drop-in replacement for `pydantic.Field()` that adds `units`, `shape`, and `example` metadata to the JSON schema
+- **`ParamInfo`**: `FieldInfo` subclass backing `Param()` — carries units/shape/example through Pydantic's schema generation
+- **`Packable.to_example()`**: Class method that builds an instance from `Param()` example/default values
+- **`Packable.with_units()`**: Returns a clone with numeric/array fields converted to `pint.Quantity` objects
+
 ### Key Capabilities
 
 - Automatic encoding/decoding of numpy array attributes via `Array`, `IndexSequence`, `InlineArray` type annotations
@@ -184,6 +195,54 @@ class OptimizedMesh(Packable):
 
 > **Note:** All dtypes are supported. Arrays with non-4-byte dtypes (e.g., `float16`, `int8`, `uint8`) are automatically padded to 4-byte alignment during encoding and unpadded during decoding (meshoptimizer requirement). For best performance, prefer 4-byte aligned dtypes like `float32`, `int32`, or `float64`.
 
+### Unit-Aware Parameters with Param()
+
+`Param()` is a drop-in replacement for `pydantic.Field()` that adds units, shape, and example metadata to the JSON schema. It works on any Pydantic `BaseModel` or `Packable` field, including `InlineArray`:
+
+```python
+from meshly import Packable, Param, InlineArray
+
+class Simulation(Packable):
+    velocity: InlineArray = Param(units="m/s", example=[30.0, 0, 0], shape=(3,),
+                                  description="Flow velocity vector [vx, vy, vz]")
+    temperature: float = Param(300.0, units="K", description="Fluid temperature")
+    pressure: float = Param(101325.0, units="Pa", description="Outlet pressure")
+    name: str = Param("default", units="dimensionless", description="Simulation name")
+
+# Units appear in the JSON schema
+schema = Simulation.model_json_schema()
+print(schema["properties"]["temperature"])
+# {'default': 300.0, 'description': 'Fluid temperature', 'title': 'Temperature',
+#  'type': 'number', 'units': 'K'}
+
+# Create from example values
+sim = Simulation.to_example()
+print(sim.velocity)     # [30.  0.  0.]
+print(sim.temperature)  # 300.0
+
+# Convert to pint Quantities (requires `pip install meshly[units]`)
+sim_units = sim.with_units()
+print(sim_units.velocity)                # [30.0 0.0 0.0] meter / second
+print(sim_units.velocity.to("km/h"))     # [108.0 0.0 0.0] kilometer / hour
+print(sim_units.temperature.to("degC"))  # 26.85 degree_Celsius
+
+# Convert to SI base units
+sim_base = sim.with_units(base_units=True)
+print(sim_base.pressure)  # 101325.0 kilogram / meter / second ** 2
+```
+
+`Param()` requires either a default value or an `example`:
+```python
+# With default
+velocity: float = Param(10.0, units="m/s")
+
+# With example (no default, field is required)
+velocity: float = Param(units="m/s", example=10.0)
+
+# Error: neither default nor example
+velocity: float = Param(units="m/s")  # ValueError!
+```
+
 ### Dict of Pydantic BaseModel Objects
 
 You can also use dictionaries containing Pydantic `BaseModel` instances with numpy arrays:
@@ -645,6 +704,34 @@ class MyData(Packable):
     color: InlineArray            # Small arrays as inline JSON (no $ref)
 ```
 
+### Param
+
+```python
+def Param(
+    default: Any = ...,
+    *,
+    units: str,                    # Required: unit string (e.g., "m/s", "Pa", "dimensionless")
+    shape: tuple[int, ...] = None, # Optional: expected array shape
+    example: Any = None,           # Optional: example value for to_example()
+    description: str = None,       # Optional: field description
+    # ... all other pydantic.Field kwargs supported (gt, ge, lt, le, etc.)
+) -> ParamInfo
+```
+
+### ParamInfo
+
+```python
+class ParamInfo(FieldInfo):
+    """FieldInfo subclass that adds units, shape, and example to the JSON schema.
+
+    Works on any Pydantic BaseModel. When used with InlineArray, the units
+    are preserved in the JSON schema output via json_schema_extra.
+    """
+    units: str
+    shape: tuple[int, ...] | None
+    example: Any
+```
+
 ### ArrayUtils
 
 ```python
@@ -702,6 +789,11 @@ class Packable(BaseModel):
     # Array conversion
     def convert_to(self, array_type: ArrayType) -> T
 
+    # Param-aware helpers
+    @classmethod
+    def to_example(cls) -> T         # Build instance from Param() example/default values
+    def with_units(self, base_units: bool = False) -> T  # Clone with pint Quantities (requires pint)
+
     # Extract/Encode (instance methods)
     def extract(self) -> ExtractedPackable  # Cached for efficiency
     def encode(self) -> bytes  # Calls extract() internally

python/meshly/__init__.py

@@ -23,6 +23,7 @@
     IndexSequence,
     InlineArray,
 )
+from meshly.param import Param, ParamInfo
 from meshly.cell_types import (
     CellType,
     CellTypeUtils,
@@ -53,6 +54,9 @@
     # Mesh classes
     "Mesh",
     "TMesh",
+    # Parameter metadata
+    "Param",
+    "ParamInfo",
     # Array types and utilities
     "Array",
     "InlineArray",

python/meshly/array.py

@@ -86,7 +86,11 @@ def __eq__(self, other):
 
 
 class _InlineArrayAnnotation:
-    """Pydantic annotation for arrays serialized as inline JSON lists."""
+    """Pydantic annotation for arrays serialized as inline JSON lists.
+
+    When the field is defined with Param(units=...), the units/shape/description
+    are preserved in the JSON schema output via json_schema_extra on the FieldInfo.
+    """
 
     def __get_pydantic_core_schema__(
         self, source_type: Any, handler: GetCoreSchemaHandler

python/meshly/packable.py

@@ -664,6 +664,97 @@ def load(
         extracted = store.load_extracted(key)
         return cls.reconstruct(extracted, assets=store.load_asset, array_type=array_type, is_lazy=is_lazy)
 
+    # -------------------------------------------------------------------------
+    # Param-aware helpers
+    # -------------------------------------------------------------------------
+
+    @classmethod
+    def to_example(cls) -> "Packable":
+        """Create an instance using example values from Param() fields.
+
+        For each Param field, uses example if defined, else falls back to default.
+        Handles both native ParamInfo fields and InlineArray fields where
+        Pydantic converts ParamInfo to plain FieldInfo.
+        """
+        from meshly.param import ParamInfo
+
+        example_data: dict[str, Any] = {}
+        for field_name, field_info in cls.model_fields.items():
+            is_param = isinstance(field_info, ParamInfo)
+            has_units_extra = (
+                isinstance(field_info.json_schema_extra, dict)
+                and "units" in field_info.json_schema_extra
+            )
+            if not is_param and not has_units_extra:
+                continue
+
+            # Try example from ParamInfo, then FieldInfo.examples, then default
+            example = None
+            if is_param and field_info.example is not None:
+                example = field_info.example
+            elif field_info.examples and len(field_info.examples) > 0:
+                example = field_info.examples[0]
+
+            if example is not None:
+                example_data[field_name] = example
+            elif field_info.default is not None and field_info.default is not ...:
+                example_data[field_name] = field_info.default
+            elif field_info.default_factory is not None:
+                example_data[field_name] = field_info.default_factory()
+            else:
+                raise ValueError(
+                    f"Parameter '{field_name}' has no example or default value. "
+                    f"Provide example=... or default=... in Param()."
+                )
+        return cls(**example_data)
+
+    def with_units(self, base_units: bool = False) -> "Packable":
+        """Clone with numeric/array Param fields converted to pint Quantities.
+
+        Reads units from either ParamInfo (for fields defined with Param()) or
+        json_schema_extra (for InlineArray fields where Pydantic converts
+        ParamInfo to plain FieldInfo but preserves the extra dict).
+
+        Args:
+            base_units: If True, convert to SI base units.
+        """
+        try:
+            from pint import UnitRegistry
+            ureg = UnitRegistry()
+        except ImportError:
+            raise ImportError("pint is required for with_units(). Install with: pip install pint")
+
+        from meshly.param import ParamInfo
+
+        cloned = self.model_copy()
+        for field_name, field_info in self.model_fields.items():
+            # Get units from ParamInfo or json_schema_extra
+            units: str | None = None
+            if isinstance(field_info, ParamInfo):
+                units = field_info.units
+            elif isinstance(field_info.json_schema_extra, dict):
+                units = field_info.json_schema_extra.get("units")
+
+            value = getattr(self, field_name)
+
+            # Recurse into nested Packable fields
+            if isinstance(value, Packable):
+                object.__setattr__(cloned, field_name, value.with_units(base_units=base_units))
+                continue
+
+            if not units or units == "dimensionless":
+                continue
+
+            try:
+                quantity = ureg.Quantity(value, units)
+                if base_units:
+                    quantity = quantity.to_base_units()
+                object.__setattr__(cloned, field_name, quantity)
+            except Exception:
+                pass
+
+        return cloned
+
     def __reduce__(self):
         """Support for pickle serialization using standard dict approach."""
         return (