fix nczarr when libnetcdf>4.8.1 (#7575)

Author: malmans2

xarray/backends/zarr.py

@@ -207,7 +207,7 @@ def _get_zarr_dims_and_attrs(zarr_obj, dimension_key, try_nczarr):
                 "which are required for xarray to determine variable dimensions."
             ) from e
 
-    nc_attrs = [attr for attr in zarr_obj.attrs if attr.startswith("_NC")]
+    nc_attrs = [attr for attr in zarr_obj.attrs if attr.lower().startswith("_nc")]
     attributes = HiddenKeyDict(zarr_obj.attrs, [dimension_key] + nc_attrs)
     return dimensions, attributes
 
@@ -495,7 +495,7 @@ def get_attrs(self):
         return {
             k: v
             for k, v in self.zarr_group.attrs.asdict().items()
-            if not k.startswith("_NC")
+            if not k.lower().startswith("_nc")
         }
 
     def get_dimensions(self):

xarray/tests/test_backends.py

@@ -5663,12 +5663,14 @@ def test_write_file_from_np_str(str_type, tmpdir) -> None:
 @requires_zarr
 @requires_netCDF4
 class TestNCZarr:
-    @staticmethod
-    def _create_nczarr(filename):
-        netcdfc_version = Version(nc4.getlibversion().split()[0])
-        if netcdfc_version < Version("4.8.1"):
+    @property
+    def netcdfc_version(self):
+        return Version(nc4.getlibversion().split()[0])
+
+    def _create_nczarr(self, filename):
+        if self.netcdfc_version < Version("4.8.1"):
             pytest.skip("requires netcdf-c>=4.8.1")
-        if (platform.system() == "Windows") and (netcdfc_version == Version("4.8.1")):
+        if platform.system() == "Windows" and self.netcdfc_version == Version("4.8.1"):
             # Bug in netcdf-c==4.8.1 (typo: Nan instead of NaN)
             # https://github.com/Unidata/netcdf-c/issues/2265
             pytest.skip("netcdf-c==4.8.1 has issues on Windows")
@@ -5678,9 +5680,7 @@ def _create_nczarr(filename):
         # https://github.com/Unidata/netcdf-c/issues/2259
         ds = ds.drop_vars("dim3")
 
-        # netcdf-c>4.8.1 will add _ARRAY_DIMENSIONS by default
-        mode = "nczarr" if netcdfc_version == Version("4.8.1") else "nczarr,noxarray"
-        ds.to_netcdf(f"file://{filename}#mode={mode}")
+        ds.to_netcdf(f"file://{filename}#mode=nczarr")
         return ds
 
     def test_open_nczarr(self) -> None:
@@ -5700,6 +5700,9 @@ def test_overwriting_nczarr(self) -> None:
     @pytest.mark.parametrize("mode", ["a", "r+"])
     @pytest.mark.filterwarnings("ignore:.*non-consolidated metadata.*")
     def test_raise_writing_to_nczarr(self, mode) -> None:
+        if self.netcdfc_version > Version("4.8.1"):
+            pytest.skip("netcdf-c>4.8.1 adds the _ARRAY_DIMENSIONS attribute")
+
         with create_tmp_file(suffix=".zarr") as tmp:
             ds = self._create_nczarr(tmp)
             with pytest.raises(