Wrap grdcut

Wrap the `grdcut` function.

Ideally it should take a grid file or a `xarray.DataArray` as input, and
store output as a grid file or return a `xarray.DataArray`.

Supported features:

- [X] input: grid file; output: grid file
- [X] input: grid file; output: `xarray.DataArray`
- [ ] input: `xarray.DataArray`; output: grid file
- [ ] input: `xarray.DataArray`; output: `xarray.DataArray`

Passing `xarray.DataArray` to `grdcut` is not implenmented in the
GMT 6.0.0. See GenericMappingTools/gmt#3532 for
a feature request to the upstream GMT.

doc/api/index.rst

@@ -72,6 +72,7 @@ Operations on grids:
 
     grdinfo
     grdtrack
+    grdcut
 
 GMT Defaults
 ------------

pygmt/__init__.py

@@ -18,7 +18,7 @@
 from .gridding import surface
 from .sampling import grdtrack
 from .mathops import makecpt
-from .modules import config, info, grdinfo, which
+from .modules import config, info, grdinfo, grdcut, which
 from . import datasets
 
 

pygmt/modules.py

@@ -1,10 +1,14 @@
 """
 Non-plot GMT modules.
 """
+import xarray as xr
+
+
 from .clib import Session
 from .helpers import (
     build_arg_string,
     fmt_docstring,
+    kwargs_to_strings,
     GMTTempFile,
     use_alias,
     data_kind,
@@ -147,6 +151,106 @@ def which(fname, **kwargs):
     return path
 
 
+@fmt_docstring
+@use_alias(
+    G="outgrid",
+    R="region",
+    J="projection",
+    N="extend",
+    S="circ_subregion",
+    Z="z_subregion",
+)
+@kwargs_to_strings(R="sequence")
+def grdcut(ingrid, **kwargs):
+    """
+    Extract subregion from a grid.
+
+    Produce a new *outgrid* file which is a subregion of *ingrid*. The
+    subregion is specified with *region*; the specified range must not exceed
+    the range of *ingrid* (but see *extend*). If in doubt, run
+    :meth:`pygmt.grdinfo` to check range. Alternatively, define the subregion
+    indirectly via a range check on the node values or via distances from a
+    given point. Finally, you can give *projection* for oblique projections to
+    determine the corresponding rectangular *region* setting that will give a
+    grid that fully covers the oblique domain.
+
+    Full option list at :gmt-docs:`grdcut.html`
+
+    {aliases}
+
+    Parameters
+    ----------
+    ingrid : str
+        The name of the input grid file.
+    outgrid : str or None
+        The name of the output netCDF file with extension .nc to store the grid
+        in.
+    {J}
+    {R}
+    extend : bool or int or float
+        Allow grid to be extended if new *region* exceeds existing boundaries.
+        Give a value to initialize nodes outside current region.
+    circ_subregion : str
+        ``'lon/lat/radius[unit][+n]'``.
+        Specify an origin (*lon* and *lat*) and *radius*; append a distance
+        *unit* and we determine the corresponding rectangular region so that
+        all grid nodes on or inside the circle are contained in the subset.
+        If **+n** is appended we set all nodes outside the circle to NaN.
+    z_subregion :
+        ``'[min/max][+n|N|r]'``.
+        Determine a new rectangular region so that all nodes outside this
+        region are also outside the given z-range [-inf/+inf]. To indicate no
+        limit on *min* or *max* only, specify a hyphen (-). Normally, any NaNs
+        encountered are simply skipped and not considered in the
+        range-decision. Append **+n** to consider a NaN to be outside the given
+        z-range. This means the new subset will be NaN-free. Alternatively,
+        append **+r** to consider NaNs to be within the data range. In this
+        case we stop shrinking the boundaries once a NaN is found [Default
+        simply skips NaNs when making the range decision]. Finally, if your
+        core subset grid is surrounded by rows and/or columns that are all
+        NaNs, append **+N** to strip off such columns before (optionally)
+        considering the range of the core subset for further reduction of the
+        area.
+
+    Returns
+    -------
+    ret: xarray.DataArray or None
+        Return type depends on whether the *outgrid* parameter is set:
+
+        - xarray.DataArray if *outgrid* is not set
+        - None if *outgrid* is set (grid output will be stored in *outgrid*)
+    """
+    kind = data_kind(ingrid)
+
+    with GMTTempFile(suffix=".nc") as tmpfile:
+        with Session() as lib:
+            if kind == "file":
+                file_context = dummy_context(ingrid)
+            elif kind == "grid":
+                raise GMTInvalidInput(
+                    "xarray.DataArray is not supported as the input grid yet!"
+                )
+                # file_context = lib.virtualfile_from_grid(ingrid)
+                # See https://github.com/GenericMappingTools/gmt/pull/3532
+                # for a feature request.
+            else:
+                raise GMTInvalidInput("Unrecognized data type: {}".format(type(ingrid)))
+
+            with file_context as infile:
+                if "G" not in kwargs.keys():  # if outgrid is unset, output to tempfile
+                    kwargs.update({"G": tmpfile.name})
+                outgrid = kwargs["G"]
+                arg_str = " ".join([infile, build_arg_string(kwargs)])
+                lib.call_module("grdcut", arg_str)
+
+        if outgrid == tmpfile.name:  # if user did not set outgrid, return DataArray
+            result = xr.open_dataarray(outgrid)
+        else:
+            result = None  # if user sets an outgrid, return None
+
+        return result
+
+
 class config:  # pylint: disable=invalid-name
     """
     Set GMT defaults globally or locally.

pygmt/tests/test_grdcut.py

@@ -0,0 +1,64 @@
+"""
+Tests for grdcut
+"""
+import os
+import numpy as np
+import pytest
+import xarray as xr
+
+from .. import grdcut, grdinfo
+from ..datasets import load_earth_relief
+from ..exceptions import GMTInvalidInput
+from ..helpers import GMTTempFile
+
+
+def test_grdcut_file_in_file_out():
+    "grduct an input grid file, and output to a grid file"
+    with GMTTempFile(suffix=".nc") as tmpfile:
+        result = grdcut("@earth_relief_01d", outgrid=tmpfile.name, region="0/180/0/90")
+        assert result is None  # return value is None
+        assert os.path.exists(path=tmpfile.name)  # check that outgrid exists
+        result = grdinfo(tmpfile.name, C=True)
+        assert result == "0 180 0 90 -8182 5651.5 1 1 180 90 1\n"
+
+
+def test_grdcut_file_in_dataarray_out():
+    "grdcut an input grid file, and output as DataArray"
+    outgrid = grdcut("@earth_relief_01d", region="0/180/0/90")
+    assert isinstance(outgrid, xr.DataArray)
+    # check information of the output grid
+    # the '@earth_relief_01d' is in pixel registration, so the grid range is
+    # not exactly 0/180/0/90
+    assert outgrid.coords["lat"].data.min() == 0.5
+    assert outgrid.coords["lat"].data.max() == 89.5
+    assert outgrid.coords["lon"].data.min() == 0.5
+    assert outgrid.coords["lon"].data.max() == 179.5
+    assert outgrid.data.min() == -8182.0
+    assert outgrid.data.max() == 5651.5
+    assert outgrid.sizes["lat"] == 90
+    assert outgrid.sizes["lon"] == 180
+
+
+def test_grdcut_dataarray_in_file_out():
+    "grdcut an input DataArray, and output to a grid file"
+    # Not supported yet.
+    # See https://github.com/GenericMappingTools/gmt/pull/3532
+
+
+def test_grdcut_dataarray_in_dataarray_out():
+    "grdcut an input DataArray, and output to a grid file"
+    # Not supported yet.
+    # See https://github.com/GenericMappingTools/gmt/pull/3532
+
+
+def test_grdcut_dataarray_in_fail():
+    "Make sure that grdcut fails correctly if DataArray is the input grid"
+    with pytest.raises(GMTInvalidInput):
+        grid = load_earth_relief()
+        grdcut(grid, region="0/180/0/90")
+
+
+def test_grdcut_fails():
+    "Check that grdcut fails correctly"
+    with pytest.raises(GMTInvalidInput):
+        grdcut(np.arange(10).reshape((5, 2)))