Implement gmt xarray BackendEntrypoint

doc/api/index.rst

@@ -197,6 +197,14 @@ Getting metadata from tabular or grid data:
     info
     grdinfo
 
+Xarray Integration
+------------------
+
+.. autosummary::
+    :toctree: generated
+
+    GMTBackendEntrypoint
+
 Enums
 -----
 

pygmt/__init__.py

@@ -65,6 +65,7 @@
     x2sys_init,
     xyz2grd,
 )
+from pygmt.xarray_backend import GMTBackendEntrypoint
 
 # Start our global modern mode session
 _begin()

pygmt/tests/test_xarray_backend.py

@@ -0,0 +1,74 @@
+"""
+Tests for xarray 'gmt' backend engine.
+"""
+
+import re
+
+import numpy as np
+import numpy.testing as npt
+import pytest
+import xarray as xr
+from pygmt.enums import GridRegistration, GridType
+from pygmt.exceptions import GMTInvalidInput
+
+
+def test_xarray_backend_gmt_open_nc_grid():
+    """
+    Ensure that passing engine='gmt' to xarray.open_dataarray works for opening NetCDF
+    grids.
+    """
+    with xr.open_dataarray(
+        "@static_earth_relief.nc", engine="gmt", raster_kind="grid"
+    ) as da:
+        assert da.sizes == {"lat": 14, "lon": 8}
+        assert da.dtype == "float32"
+        assert da.gmt.registration == GridRegistration.PIXEL
+        assert da.gmt.gtype == GridType.GEOGRAPHIC
+
+
+def test_xarray_backend_gmt_open_tif_image():
+    """
+    Ensure that passing engine='gmt' to xarray.open_dataarray works for opening GeoTIFF
+    images.
+    """
+    with xr.open_dataarray("@earth_day_01d", engine="gmt", raster_kind="image") as da:
+        assert da.sizes == {"band": 3, "y": 180, "x": 360}
+        assert da.dtype == "uint8"
+        assert da.gmt.registration == GridRegistration.PIXEL
+        assert da.gmt.gtype == GridType.GEOGRAPHIC
+
+
+def test_xarray_backend_gmt_load_grd_grid():
+    """
+    Ensure that passing engine='gmt' to xarray.load_dataarray works for loading GRD
+    grids.
+    """
+    da = xr.load_dataarray(
+        "@earth_relief_20m_holes.grd", engine="gmt", raster_kind="grid"
+    )
+    # Ensure data is in memory.
+    assert isinstance(da.data, np.ndarray)
+    npt.assert_allclose(da.min(), -4929.5)
+    assert da.sizes == {"lat": 31, "lon": 31}
+    assert da.dtype == "float32"
+    assert da.gmt.registration == GridRegistration.GRIDLINE
+    assert da.gmt.gtype == GridType.GEOGRAPHIC
+
+
+def test_xarray_backend_gmt_read_invalid_kind():
+    """
+    Check that xarray.open_dataarray(..., engine="gmt") fails with missing or incorrect
+    'raster_kind'.
+    """
+    with pytest.raises(
+        TypeError,
+        match=re.escape(
+            "GMTBackendEntrypoint.open_dataset() missing 1 required keyword-only argument: 'raster_kind'"
+        ),
+    ):
+        xr.open_dataarray("nokind.nc", engine="gmt")
+
+    with pytest.raises(GMTInvalidInput):
+        xr.open_dataarray(
+            filename_or_obj="invalid.tif", engine="gmt", raster_kind="invalid"
+        )

pygmt/xarray_backend.py

@@ -0,0 +1,106 @@
+"""
+An xarray backend for reading raster grid/image files using the 'gmt' engine.
+"""
+
+from typing import Literal
+
+import xarray as xr
+from pygmt._typing import PathLike
+from pygmt.clib import Session
+from pygmt.exceptions import GMTInvalidInput
+from pygmt.helpers import build_arg_list
+from pygmt.src.which import which
+from xarray.backends import BackendEntrypoint
+
+
+class GMTBackendEntrypoint(BackendEntrypoint):
+    """
+    Xarray backend to read raster grid/image files using 'gmt' engine.
+
+    Internally, GMT uses the NetCDF C library to read NetCDF files, and GDAL for GeoTIFF
+    and other raster formats.
+
+    When using :py:func:`xarray.open_dataarray` or :py:func:`xarray.load_dataarray` with
+    ``engine="gmt"``, pass the ``raster_kind`` parameter that can be either:
+
+    - ``"grid"`` - for reading single-band raster grids
+    - ``"image"`` - for reading multi-band raster images
+
+    Examples
+    --------
+    Read a single-band NetCDF file using ``raster_kind="grid"``
+
+    >>> import pygmt
+    >>> import xarray as xr
+    >>>
+    >>> da_grid = xr.open_dataarray(
+    ...     "@static_earth_relief.nc", engine="gmt", raster_kind="grid"
+    ... )
+    >>> da_grid  # doctest: +NORMALIZE_WHITESPACE, +ELLIPSIS
+    <xarray.DataArray 'z' (lat: 14, lon: 8)>...
+    [112 values with dtype=float32]
+    Coordinates:
+      * lat      (lat) float64... -23.5 -22.5 -21.5 -20.5 ... -12.5 -11.5 -10.5
+      * lon      (lon) float64... -54.5 -53.5 -52.5 -51.5 -50.5 -49.5 -48.5 -47.5
+    Attributes:
+        Conventions:   CF-1.7
+        title:         Produced by grdcut
+        history:       grdcut @earth_relief_01d_p -R-55/-47/-24/-10 -Gstatic_eart...
+        description:   Reduced by Gaussian Cartesian filtering (111.2 km fullwidt...
+        actual_range:  [190. 981.]
+        long_name:     elevation (m)
+
+    Read a multi-band GeoTIFF file using ``raster_kind="image"``
+
+    >>> da_image = xr.open_dataarray(
+    ...     "@earth_night_01d", engine="gmt", raster_kind="image"
+    ... )
+    >>> da_image  # doctest: +NORMALIZE_WHITESPACE, +ELLIPSIS
+    <xarray.DataArray 'z' (band: 3, y: 180, x: 360)>...
+    [194400 values with dtype=uint8]
+    Coordinates:
+      * y        (y) float64... 89.5 88.5 87.5 86.5 ... -86.5 -87.5 -88.5 -89.5
+      * x        (x) float64... -179.5 -178.5 -177.5 -176.5 ... 177.5 178.5 179.5
+      * band     (band) uint8... 1 2 3
+    Attributes:
+        long_name:  z
+    """
+
+    description = "Open raster (.grd, .nc or .tif) files in Xarray via GMT."
+    open_dataset_parameters = ("filename_or_obj", "raster_kind")
+    url = "https://pygmt.org/dev/api/generated/pygmt.GMTBackendEntrypoint.html"
+
+    def open_dataset(  # type: ignore[override]
+        self,
+        filename_or_obj: PathLike,
+        *,
+        drop_variables=None,  # noqa: ARG002
+        raster_kind: Literal["grid", "image"],
+        # other backend specific keyword arguments
+        # `chunks` and `cache` DO NOT go here, they are handled by xarray
+    ) -> xr.Dataset:
+        """
+        Backend open_dataset method used by Xarray in :py:func:`~xarray.open_dataset`.
+        """
+        if raster_kind not in {"grid", "image"}:
+            msg = f"Invalid raster kind: '{raster_kind}'. Valid values are 'grid' or 'image'."
+            raise GMTInvalidInput(msg)
+
+        with Session() as lib:
+            with lib.virtualfile_out(kind=raster_kind) as voutfile:
+                kwdict = {"T": {"grid": "g", "image": "i"}[raster_kind]}
+                lib.call_module(
+                    module="read",
+                    args=[filename_or_obj, voutfile, *build_arg_list(kwdict)],
+                )
+
+                raster: xr.DataArray = lib.virtualfile_to_raster(
+                    vfname=voutfile, kind=raster_kind
+                )
+                # Add "source" encoding
+                source = which(fname=filename_or_obj)
+                raster.encoding["source"] = (
+                    source[0] if isinstance(source, list) else source
+                )
+                _ = raster.gmt  # Load GMTDataArray accessor information
+                return raster.to_dataset()

pyproject.toml

@@ -52,6 +52,9 @@ all = [
     "rioxarray",
 ]
 
+[project.entry-points."xarray.backends"]
+gmt = "pygmt.xarray_backend:GMTBackendEntrypoint"
+
 [project.urls]
 "Homepage" = "https://www.pygmt.org"
 "Documentation" = "https://www.pygmt.org"