Make Grid.geometry_type an argument, not derived

This means that users can check the geometry type before building the
geometry, which can help with resource usage in some cases.

Author: mx-moth

src/emsarray/conventions/_base.py

@@ -62,6 +62,9 @@ class Grid[GridKind, Index](abc.ABC):
     #: The :type:`GridKind` this grid represents
     grid_kind: GridKind
 
+    #: A Shapely geometry class such as :class:`shapely.Polygon` or :class:`shapely.Point`.
+    geometry_type: type[BaseGeometry]
+
     @property
     @abc.abstractmethod
     def size(self) -> int:
@@ -87,13 +90,6 @@ def geometry(self) -> numpy.ndarray:
         """
         return self.convention.make_geometry(self.grid_kind)
 
-    @cached_property
-    def geometry_type(self) -> type[BaseGeometry]:
-        """
-        A Shapely geometry class, such as :class:`shapely.Polygon` or :class:`shapely.Point`.
-        """
-        return type(next(shape for shape in self.geometry if shape is not None))
-
     @cached_property
     def strtree(self) -> STRtree:
         """
@@ -2186,7 +2182,9 @@ class DimensionConvention[GridKind](Convention[GridKind, DimensionIndex[GridKind
     are always defined on unique sets of dimension.
     This covers most conventions.
 
-    This subclass adds the abstract property :attr:`.grid_dimensions` that subclasses must define.
+    This subclass adds the abstract properties
+    :attr:`.grid_dimensions` and :attr:`.geometry_types`
+    that subclasses must define.
 
     Default implementations are provided for all abstract :class:`Grid` methods
     using :class:`DimensionGrid`.
@@ -2199,6 +2197,7 @@ def grids(self) -> dict[GridKind, Grid[GridKind, DimensionIndex[GridKind]]]:
                 convention=self,
                 grid_kind=grid_kind,
                 dimensions=self.grid_dimensions[grid_kind],
+                geometry_type=self.geometry_types[grid_kind],
             )
             for grid_kind in self.grid_kinds
         }
@@ -2222,6 +2221,21 @@ def grid_dimensions(self) -> dict[GridKind, Sequence[Hashable]]:
         """
         pass
 
+    @property
+    @abc.abstractmethod
+    def geometry_types(self) -> dict[GridKind, type[BaseGeometry]]:
+        """
+        The geometry type associated with a particular grid kind.
+
+        This is a mapping between :type:`grid kinds <GridKind>`
+        and shapely geometry types such as
+        :class:`~shapely.Polygon` and :class:`~shapely.Point`.
+
+        Users should use :attr:`Grid.geometry_type` instead of this property,
+        this is only used to construct the grids.
+        """
+        pass
+
     @property
     @utils.deprecated(
         "DimensionConvention.grid_shape[grid_kind] is deprecated. "

src/emsarray/conventions/arakawa_c.py

@@ -158,6 +158,12 @@ class ArakawaC(DimensionConvention[ArakawaCGridKind]):
     """
     grid_kinds = frozenset(ArakawaCGridKind)
     default_grid_kind = ArakawaCGridKind.face
+    geometry_types = {
+        ArakawaCGridKind.face: shapely.Polygon,
+        ArakawaCGridKind.left: shapely.LineString,
+        ArakawaCGridKind.back: shapely.LineString,
+        ArakawaCGridKind.node: shapely.Point,
+    }
 
     coordinate_names: ArakawaCCoordinates
 

src/emsarray/conventions/grid.py

@@ -192,6 +192,10 @@ class CFGrid[Topology: CFGridTopology](DimensionConvention[CFGridKind]):
 
     grid_kinds = frozenset(CFGridKind)
     default_grid_kind = CFGridKind.face
+    geometry_types = {
+        CFGridKind.face: shapely.Polygon,
+    }
+
     topology_class: type[Topology]
 
     def __init__(

src/emsarray/conventions/ugrid.py

@@ -1039,6 +1039,11 @@ class UGrid(DimensionConvention[UGridKind]):
     """
 
     default_grid_kind = UGridKind.face
+    geometry_types = {
+        UGridKind.face: shapely.Polygon,
+        UGridKind.edge: shapely.LineString,
+        UGridKind.node: shapely.Point,
+    }
 
     @classmethod
     def check_dataset(cls, dataset: xarray.Dataset) -> int | None:

tests/conventions/test_base.py

@@ -104,7 +104,7 @@ def shape(self) -> tuple[int, int]:
 
     @cached_property
     def grids(self) -> dict[SimpleGridKind, Grid[SimpleGridKind, SimpleGridIndex]]:
-        return {SimpleGridKind.face: SimpleGrid(self, SimpleGridKind.face)}
+        return {SimpleGridKind.face: SimpleGrid(self, SimpleGridKind.face, Polygon)}
 
     def get_grid_kind(self, data_array: xarray.DataArray) -> SimpleGridKind:
         if set(data_array.dims) >= {'x', 'y'}: