Wrap the GMT API function GMT_Read_Data to read data into GMT data containers

doc/api/index.rst

@@ -309,6 +309,7 @@ Low level access (these are mostly used by the :mod:`pygmt.clib` package):
     clib.Session.put_matrix
     clib.Session.put_strings
     clib.Session.put_vector
+    clib.Session.read_data
     clib.Session.write_data
     clib.Session.open_virtualfile
     clib.Session.read_virtualfile

pygmt/clib/conversion.py

@@ -247,7 +247,9 @@ def as_c_contiguous(array):
     return array
 
 
-def sequence_to_ctypes_array(sequence: Sequence, ctype, size: int) -> ctp.Array | None:
+def sequence_to_ctypes_array(
+    sequence: Sequence | None, ctype, size: int
+) -> ctp.Array | None:
     """
     Convert a sequence of numbers into a ctypes array variable.
 

pygmt/clib/session.py

@@ -10,6 +10,7 @@
 import pathlib
 import sys
 import warnings
+from collections.abc import Sequence
 from typing import Literal
 
 import numpy as np
@@ -1068,6 +1069,75 @@ def put_matrix(self, dataset, matrix, pad=0):
         if status != 0:
             raise GMTCLibError(f"Failed to put matrix of type {matrix.dtype}.")
 
+    def read_data(
+        self,
+        infile: str,
+        kind: Literal["dataset", "grid"],
+        mode: str | None = None,
+        wesn: Sequence[float] | None = None,
+        data=None,
+    ):
+        """
+        Read a data file into a GMT data container.
+
+        Wraps ``GMT_Read_Data`` but only allows reading from a file. The function
+        definition is different from the original C API function.
+
+        Parameters
+        ----------
+        input
+            The input file name.
+        kind
+            The data kind of the input file. Currently, valid values are ``"dataset"``
+            and ``"grid"``.
+        mode
+            How the data is to be read from the file. This option varies depending on
+            the given family. See the GMT API documentation for details.
+        wesn
+            Subregion of the data, in the form of [xmin, xmax, ymin, ymax, zmin, zmax].
+            If ``None``, the whole data is read.
+        data
+            ``None`` or the pointer returned by this function after a first call. It's
+            useful when reading grids in two steps (get a grid structure with a header,
+            then read the data).
+
+        Returns
+        -------
+        Pointer to the data container, or ``None`` if there were errors.
+        """
+        c_read_data = self.get_libgmt_func(
+            "GMT_Read_Data",
+            argtypes=[
+                ctp.c_void_p,  # V_API
+                ctp.c_uint,  # family
+                ctp.c_uint,  # method
+                ctp.c_uint,  # geometry
+                ctp.c_uint,  # mode
+                ctp.POINTER(ctp.c_double),  # wesn
+                ctp.c_char_p,  # infile
+                ctp.c_void_p,  # data
+            ],
+            restype=ctp.c_void_p,  # data_ptr
+        )
+
+        # Determine the family and geometry from kind
+        family, geometry, dtype = {
+            "dataset": ("GMT_IS_DATASET", "GMT_IS_PLP", _GMT_DATASET),
+            "grid": ("GMT_IS_GRID", "GMT_IS_SURFACE", _GMT_GRID),
+        }[kind]
+
+        data_ptr = c_read_data(
+            self.session_pointer,
+            self[family],
+            self["GMT_IS_FILE"],  # Reading from a file
+            self[geometry],
+            self["GMT_READ_NORMAL"] if mode is None else self[mode],
+            sequence_to_ctypes_array(wesn, ctp.c_double, 6),
+            infile.encode(),
+            data,
+        )
+        return ctp.cast(data_ptr, ctp.POINTER(dtype))
+
     def write_data(self, family, geometry, mode, wesn, output, data):
         """
         Write a GMT data container to a file.

pygmt/tests/test_clib_read_data.py

@@ -0,0 +1,129 @@
+"""
+Test the Session.read_data method.
+"""
+
+from pathlib import Path
+
+import pandas as pd
+import pytest
+import xarray as xr
+from pygmt.clib import Session
+from pygmt.helpers import GMTTempFile
+from pygmt.io import load_dataarray
+from pygmt.src import which
+
+try:
+    import rioxarray  # noqa: F401
+
+    _HAS_RIOXARRAY = True
+except ImportError:
+    _HAS_RIOXARRAY = False
+
+
+@pytest.fixture(scope="module", name="expected_xrgrid")
+def fixture_expected_xrgrid():
+    """
+    The expected xr.DataArray object for the static_earth_relief.nc file.
+    """
+    return load_dataarray(which("@static_earth_relief.nc"))
+
+
+def test_clib_read_data_dataset():
+    """
+    Test the Session.read_data method for datasets.
+    """
+    with GMTTempFile(suffix=".txt") as tmpfile:
+        # Prepare the sample data file
+        with Path(tmpfile.name).open(mode="w", encoding="utf-8") as fp:
+            print("# x y z name", file=fp)
+            print(">", file=fp)
+            print("1.0 2.0 3.0 TEXT1 TEXT23", file=fp)
+            print("4.0 5.0 6.0 TEXT4 TEXT567", file=fp)
+            print(">", file=fp)
+            print("7.0 8.0 9.0 TEXT8 TEXT90", file=fp)
+            print("10.0 11.0 12.0 TEXT123 TEXT456789", file=fp)
+
+        with Session() as lib:
+            ds = lib.read_data(tmpfile.name, kind="dataset").contents
+            df = ds.to_dataframe(header=0)
+            expected_df = pd.DataFrame(
+                data={
+                    "x": [1.0, 4.0, 7.0, 10.0],
+                    "y": [2.0, 5.0, 8.0, 11.0],
+                    "z": [3.0, 6.0, 9.0, 12.0],
+                    "name": pd.Series(
+                        [
+                            "TEXT1 TEXT23",
+                            "TEXT4 TEXT567",
+                            "TEXT8 TEXT90",
+                            "TEXT123 TEXT456789",
+                        ],
+                        dtype=pd.StringDtype(),
+                    ),
+                }
+            )
+            pd.testing.assert_frame_equal(df, expected_df)
+
+
+def test_clib_read_data_grid(expected_xrgrid):
+    """
+    Test the Session.read_data method for grids.
+    """
+    with Session() as lib:
+        grid = lib.read_data("@static_earth_relief.nc", kind="grid").contents
+        xrgrid = grid.to_dataarray()
+        xr.testing.assert_equal(xrgrid, expected_xrgrid)
+        assert grid.header.contents.n_bands == 1  # Explicitely check n_bands
+
+
+def test_clib_read_data_grid_two_steps(expected_xrgrid):
+    """
+    Test the Session.read_data method for grids in two steps, first reading the header
+    and then the data.
+    """
+    infile = "@static_earth_relief.nc"
+    with Session() as lib:
+        # Read the header first
+        data_ptr = lib.read_data(infile, kind="grid", mode="GMT_CONTAINER_ONLY")
+        grid = data_ptr.contents
+        header = grid.header.contents
+        assert header.n_rows == 14
+        assert header.n_columns == 8
+        assert header.wesn[:] == [-55.0, -47.0, -24.0, -10.0]
+        assert header.z_min == 190.0
+        assert header.z_max == 981.0
+        assert header.n_bands == 1  # Explicitely check n_bands
+        assert not grid.data  # The data is not read yet
+
+        # Read the data
+        lib.read_data(infile, kind="grid", mode="GMT_DATA_ONLY", data=data_ptr)
+        xrgrid = data_ptr.contents.to_dataarray()
+        xr.testing.assert_equal(xrgrid, expected_xrgrid)
+
+
+def test_clib_read_data_grid_actual_image():
+    """
+    Test the Session.read_data method for grid, but actually the file is an image.
+    """
+    with Session() as lib:
+        data_ptr = lib.read_data(
+            "@earth_day_01d_p", kind="grid", mode="GMT_CONTAINER_AND_DATA"
+        )
+        image = data_ptr.contents
+        header = image.header.contents
+        assert header.n_rows == 180
+        assert header.n_columns == 360
+        assert header.wesn[:] == [-180.0, 180.0, -90.0, 90.0]
+        # Explicitely check n_bands. Only one band is read for 3-band images.
+        assert header.n_bands == 1
+
+        if _HAS_RIOXARRAY:  # Full check if rioxarray is installed.
+            xrimage = image.to_dataarray()
+            expected_xrimage = xr.open_dataarray(which("@earth_day_01d_p"))
+            assert expected_xrimage.band.size == 3  # 3-band image.
+            xr.testing.assert_equal(
+                xrimage,
+                expected_xrimage.isel(band=0)
+                .drop_vars(["band", "spatial_ref"])
+                .sortby("y"),
+            )