Explicitly use netcdf4 engine in xarray.open_dataarray to read grd files (#1264)

Using xr.open_datarray(..., engine="netcdf4") to read
*.grd files, especially for the load_earth_relief function.
Needed for xarray v0.18 due to backend API changes.

Author: weiji14

pygmt/datasets/earth_relief.py

@@ -133,7 +133,7 @@ def load_earth_relief(resolution="01d", region=None, registration=None, use_srtm
                 f"'region' is required for Earth relief resolution '{resolution}'."
             )
         fname = which(f"@earth_relief_{resolution}{reg}", download="a")
-        with xr.open_dataarray(fname) as dataarray:
+        with xr.open_dataarray(fname, engine="netcdf4") as dataarray:
             grid = dataarray.load()
             _ = grid.gmt  # load GMTDataArray accessor information
     else:

pygmt/tests/test_accessor.py

@@ -24,7 +24,7 @@ def test_accessor_pixel_geographic():
     and a gtype value of 0 when using Cartesian coordinates.
     """
     fname = which(fname="@earth_relief_01d_p", download="a")
-    grid = xr.open_dataarray(fname)
+    grid = xr.open_dataarray(fname, engine="netcdf4")
     assert grid.gmt.registration == 1  # pixel registration
     assert grid.gmt.gtype == 1  # geographic coordinate type
 

pygmt/tests/test_clib_put_matrix.py

@@ -92,7 +92,7 @@ def test_put_matrix_grid():
                 npt.assert_allclose(newdata, data)
 
             # Save the data to a netCDF grid and check that xarray can load it
-            with GMTTempFile() as tmp_grid:
+            with GMTTempFile(suffix=".nc") as tmp_grid:
                 lib.write_data(
                     "GMT_IS_MATRIX",
                     "GMT_IS_SURFACE",