Fix silent byte-swapping of non-native-endian attribute values in the netcdf4 backend

doc/whats-new.rst

@@ -53,6 +53,10 @@ Deprecations
 Bug Fixes
 ~~~~~~~~~
 
+- Fix the ``netcdf4`` engine silently writing byte-swapped values for
+  non-native-endian numeric attribute arrays, such as attributes of netCDF-3
+  files read with the ``scipy`` engine (:pull:`11543`).
+  By `glaziermag <https://github.com/glaziermag>`_.
 - Fix async zarr tests using ``wraps`` with ``autospec=True`` on async methods,
   which caused ``AsyncMock`` objects to leak through instead of real array data
   (:pull:`11232`).

xarray/backends/netCDF4_.py

@@ -252,6 +252,15 @@ def _force_native_endianness(var):
     return var
 
 
+def _force_native_endianness_attr(value):
+    # netCDF4-python writes non-native-endian attribute arrays without
+    # byte-swapping, silently corrupting the stored values (variable data
+    # is handled by _force_native_endianness above).
+    if isinstance(value, np.ndarray) and value.dtype.byteorder not in ("=", "|"):
+        value = value.astype(value.dtype.newbyteorder("="))
+    return value
+
+
 def _extract_nc4_variable_encoding(
     variable: Variable,
     raise_on_invalid=False,
@@ -632,6 +641,7 @@ def set_dimension(self, name, length, is_unlimited=False):
         self.ds.createDimension(name, size=dim_length)
 
     def set_attribute(self, key, value):
+        value = _force_native_endianness_attr(value)
         if self.format != "NETCDF4":
             value = encode_nc3_attr_value(value)
         if _is_list_of_strings(value):
@@ -652,7 +662,7 @@ def prepare_variable(
         self, name, variable: Variable, check_encoding=False, unlimited_dims=None
     ):
         _ensure_no_forward_slash_in_name(name)
-        attrs = variable.attrs.copy()
+        attrs = {k: _force_native_endianness_attr(v) for k, v in variable.attrs.items()}
         fill_value = attrs.pop("_FillValue", None)
         datatype: np.dtype | ncEnumType | h5EnumType
         datatype = _get_datatype(

xarray/tests/test_backends.py

@@ -7252,6 +7252,31 @@ def test_load_single_value_h5netcdf(tmp_path: Path) -> None:
         ds2["test"][0].load()
 
 
+@requires_netCDF4
+def test_roundtrip_non_native_endian_attrs(tmp_path: Path) -> None:
+    """Test that non-native-endian numeric attribute values round-trip.
+
+    The scipy backend returns attribute values of netCDF-3 files as big-endian
+    arrays; netCDF4-python does not byte-swap attribute arrays on write, so
+    writing them back with the netCDF4 backend silently stored byte-swapped
+    values (e.g. valid_range [0.0, 1.0] became [0.0, 3.03865e-319]).
+    """
+    ds = xr.Dataset(
+        {
+            "x": xr.DataArray(
+                [1.0],
+                dims=("d",),
+                attrs={"valid_range": np.array([0.0, 1.0], dtype=">f8")},
+            )
+        },
+        attrs={"levels": np.array([1, 2], dtype=">i4")},
+    )
+    ds.to_netcdf(tmp_path / "test.nc", engine="netcdf4")
+    with xr.open_dataset(tmp_path / "test.nc", engine="netcdf4") as ds2:
+        assert_array_equal(ds2["x"].attrs["valid_range"], [0.0, 1.0])
+        assert_array_equal(ds2.attrs["levels"], [1, 2])
+
+
 @requires_zarr
 @requires_dask
 @pytest.mark.parametrize(