zarr-cm-tree

GeoZarr convention attribute builders — an opinionated bridge between pyproj and zarr-cm.

It produces the plain attribute dicts that make up the attributes block of a Zarr v3 group or array for the GeoZarr conventions proj:, spatial: and multiscales, plus an interim dggs: placeholder.

uv add zarr-cm-tree

Scope

zarr_cm owns the conventions themselves: schemas, validation, revisions, and the zarr_conventions registry entries. This package adds only what it deliberately does not:

• A pyproj bridge. GeoZarrProj holds a canonical pyproj.CRS and decides how to serialise it — authority code when one exists, WKT2 otherwise — and hands rioxarray a matching CRS string via rio_input().
• Domain defaults for gridded data. GeoZarrSpatial defaults to ("y", "x") dims and "pixel" registration, and infers transform_type="affine".
• Layout rules the schema does not encode, e.g. derived_from requires a transform.
• geozarr_attrs, which emits several conventions at once together with a matching zarr_conventions registry array, so the registry always describes exactly the conventions present.

Everything returned is a plain dict, never a model, so it merges straight into xarray.Dataset.attrs.

Usage

import zarr_cm_tree as gz

crs = gz.GeoZarrProj.from_user_input("EPSG:27704")  # Equi7Grid EU
spatial = gz.GeoZarrSpatial(
    transform=(10.0, 0.0, 5400000.0, 0.0, -10.0, 2700000.0),
    shape=(10000, 10000),
)

attrs = gz.geozarr_attrs(crs=crs, spatial=spatial)
# {"proj:code": "EPSG:27704", "spatial:transform": [...], "zarr_conventions": [...]}

group.attrs.update(attrs)  # or ds.attrs.update(attrs)

Add a pyramid by passing the layout, one entry per level:

attrs = gz.geozarr_attrs(
    crs=crs,
    multiscales=[
        {"asset": "0"},
        {"asset": "1", "derived_from": "0", "transform": {"scale": [2.0, 2.0]}},
    ],
)

The individual builders (build_proj_attrs, build_spatial_attrs, build_multiscales_attrs, build_zarr_conventions, merge_attrs) are public too, for callers that want to assemble attrs themselves. build_zarr_conventions accepts the legacy "geo-proj" spelling as an alias for "proj", since zarr_cm.ConventionName still carries it and so do stores written before the upstream rename.

Convention revisions

Each convention in zarr_cm ships at one or more numbered revisions. A revision fixes the convention's JSON Schema: which attribute keys are legal, how strictly they are validated, and the schema_url written into the store's zarr_conventions array.

This package writes at whatever revision the installed zarr_cm declares latest, rather than pinning one — a pin nobody updates goes stale, and every store records the revision that wrote it in its own schema_url, so drift stays auditable after the fact. Reading is unaffected either way: zarr_cm.<convention>.detect() sniffs the revision from the store.

current_revisions() reports what is in use:

Convention	Current	Notes
proj:	r3	r2 restricted proj:code to ^[A-Z]+:[0-9]+$, so OGC:CRS84 was invalid
spatial:	r3	same keys as r2, looser validation
multiscales	r2	the only revision published so far

tests/test_conventions.py asserts these, so a zarr_cm upgrade that ships a new revision surfaces as one failing test rather than silently changing what gets written. When it fails: read what changed, confirm the new attributes are what you want, update EXPECTED_REVISIONS. Stricter validation would fail loudly at ingest anyway; the test is there for the revisions that change quietly.

Known issues

• Writing a CRS onto a group with no y/x spatial dims (e.g. HEALPix) produces a bogus identity GeoTransform. Carried over from cube-factory (ZarrGroup.to_dataset in _core.py); the fix belongs wherever the rioxarray-facing write path ends up living.

Development

uv sync --all-groups
just test        # pytest
just check       # ruff lint + format + ty