Ensure surface and grdcut loads GMTDataArray accessor info into xarray (

#539)

* Ensure surface and grdcut loads GMTDataArray accessor info into xarray

For modules like `surface` and `grdcut` that output an xarray.DataArray grid, we need to ensure that the registration and gtype information is loaded properly into the xarray.DataArray before the temporary netcdf file gets deleted.

* Fix pylint pointless-statement warning

pygmt/gridding.py

@@ -90,6 +90,7 @@ def surface(x=None, y=None, z=None, data=None, **kwargs):
         if outfile == tmpfile.name:  # if user did not set outfile, return DataArray
             with xr.open_dataarray(outfile) as dataarray:
                 result = dataarray.load()
+                _ = result.gmt  # load GMTDataArray accessor information
         elif outfile != tmpfile.name:  # if user sets an outfile, return None
             result = None
 

pygmt/gridops.py

@@ -113,6 +113,7 @@ def grdcut(grid, **kwargs):
         if outgrid == tmpfile.name:  # if user did not set outgrid, return DataArray
             with xr.open_dataarray(outgrid) as dataarray:
                 result = dataarray.load()
+                _ = result.gmt  # load GMTDataArray accessor information
         else:
             result = None  # if user sets an outgrid, return None
 

pygmt/tests/test_grdcut.py

@@ -26,6 +26,8 @@ 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)
+    assert outgrid.gmt.registration == 1  # Pixel registration
+    assert outgrid.gmt.gtype == 1  # Geographic type
     # 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

pygmt/tests/test_surface.py

@@ -23,6 +23,8 @@ def test_surface_input_file():
     fname = which("@tut_ship.xyz", download="c")
     output = surface(data=fname, spacing="5m", region=[245, 255, 20, 30])
     assert isinstance(output, xr.DataArray)
+    assert output.gmt.registration == 0  # Gridline registration
+    assert output.gmt.gtype == 0  # Cartesian type
     return output