feat(geoarrow-types): pyarrow geometry extension type implementation

[paleolimbot]

import geoarrow.types as gt

extension_type = gt.wkb(crs=gt.OGC_CRS84).to_pyarrow()
extension_type.storage_type
#> DataType(binary)

extension_type.crs
#> ProjJsonCrs(OGC:CRS84)

extension_type.to_pandas_dtype()
#> extension<geoarrow.wkb<WkbType>>[pyarrow]

extension_type = gt.point(dimensions="xyz", coord_type="interleaved").to_pyarrow()
extension_type.storage_type
#> FixedSizeListType(fixed_size_list<xyz: double not null>[3])

[kylebarron]

I'm not 100% clear on the difference between unspecified and unknown

[paleolimbot]

The intention is that it's like GEOMETRY (i.e., potentially mixed but we don't know) but for dimensions. I added it here because I realized that gt.wkb().to_pyarrow().dimensions was returning UNSPECIFIED, which is not really an accurate description of what's happening.

It could be argued that UNSPECIFIED should just be None, and it could be implemented that way (but makes the typing slightly more awkward). In the case of crs, None is a perfectly acceptable value, so there at least needs to be an UNSPECIFIED sentinel for that one (such that coalesce(None, crs.UNSPECIFIED -> None).

The intention is that users don't run into UNSPECIFIED all that often (they just generate it by implicitly not adding a type constraint).

import geoarrow.types as gt

gt.TypeSpec.coalesce(gt.GeometryType.UNSPECIFIED, gt.GeometryType.POINT)
#> TypeSpec(GeometryType.POINT)
gt.TypeSpec.coalesce(gt.GeometryType.GEOMETRY, gt.GeometryType.POINT)
#> TypeSpec(GeometryType.GEOMETRY)

gt.TypeSpec.coalesce(gt.Dimensions.UNSPECIFIED, gt.Dimensions.XYZ)
#> TypeSpec(Dimensions.XYZ)
gt.TypeSpec.coalesce(gt.Dimensions.UNKNOWN, gt.Dimensions.XYZ)
#> TypeSpec(Dimensions.UNKNOWN)

gt.TypeSpec.common(gt.Dimensions.XYM, gt.Dimensions.XYZ)
#> TypeSpec(Dimensions.XYZM)

gt.TypeSpec.common(gt.Dimensions.UNSPECIFIED, gt.Dimensions.XYZ)
#> TypeSpec(Dimensions.XYZ)

gt.TypeSpec.common(gt.Dimensions.UNKNOWN, gt.Dimensions.XYZ)
#> TypeSpec(Dimensions.UNKNOWN)

[kylebarron]

As an aside, I do like the ideas of coalesce and common for types. I had so much pain working with type casting in geoarrow-rs

[paleolimbot]

I stole the idea from the R vctrs package! https://vctrs.r-lib.org/reference/internal-faq-ptype2-identity.html