Reject ambiguous MultiIndex names in CF compression

cf_xarray/coding.py

@@ -23,6 +23,12 @@ def encode_multi_index_as_compress(ds, idxnames=None):
     xarray.Dataset
         Encoded Dataset with ``name`` as a integer coordinate with a ``"compress"`` attribute.
 
+    Raises
+    ------
+    ValueError
+        If a MultiIndex level name is empty, is not a string, or contains
+        whitespace, which cannot be represented in the CF ``compress`` attribute.
+
     References
     ----------
     CF conventions on `compression by gathering <http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#compression-by-gathering>`_
@@ -43,6 +49,14 @@ def encode_multi_index_as_compress(ds, idxnames=None):
     if not idxnames:
         raise ValueError("No MultiIndex-ed dimensions found in Dataset.")
 
+    for idxname in idxnames:
+        for name in ds.indexes[idxname].names:
+            if not isinstance(name, str) or not name or any(c.isspace() for c in name):
+                raise ValueError(
+                    "MultiIndex level names must be nonempty strings without whitespace "
+                    f"to be encoded in a CF 'compress' attribute. Got {name!r}."
+                )
+
     encoded = ds.reset_index(idxnames)
     for idxname in idxnames:
         mindex = ds.indexes[idxname]

cf_xarray/tests/test_coding.py

@@ -6,6 +6,24 @@
 import cf_xarray as cfxr
 
 
+@pytest.mark.parametrize(
+    "name", ["lat name", " leading", "trailing ", "tab\tname", "line\nname", ""]
+)
+@pytest.mark.parametrize("level", [0, 1])
+def test_compression_rejects_ambiguous_level_names(name, level):
+    names = ["lat", "lon"]
+    names[level] = name
+    index = pd.MultiIndex.from_product([[1, 2], [3, 4]], names=names)
+    dataset = xr.Dataset(
+        {"data": ("point", [1, 2, 3, 4])},
+        coords=xr.Coordinates.from_pandas_multiindex(index, "point"),
+    )
+    before = dataset.copy(deep=True)
+    with pytest.raises(ValueError, match="nonempty strings without whitespace"):
+        cfxr.encode_multi_index_as_compress(dataset)
+    xr.testing.assert_identical(dataset, before)
+
+
 @pytest.mark.parametrize("with_data", [False, True])
 def test_compression_preserves_unrelated_coordinates(with_data):
     index = pd.MultiIndex.from_product([[1, 2], [3, 4]], names=("x", "y"))