Fix multiindex level serialization after reset_index (pydata#8672)

* fix serialize multi-index level coord after reset

* add regression test

* update what's new

---------

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

Author: benbovy

doc/whats-new.rst

@@ -43,6 +43,9 @@ Deprecations
 Bug fixes
 ~~~~~~~~~
 
+- Fixed a regression that prevented multi-index level coordinates being
+  serialized after resetting or dropping the multi-index (:issue:`8628`, :pull:`8672`).
+  By `Benoit Bovy <https://github.com/benbovy>`_.
 - Fix bug with broadcasting when wrapping array API-compliant classes. (:issue:`8665`, :pull:`8669`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
 - Ensure :py:meth:`DataArray.unstack` works when wrapping array API-compliant classes. (:issue:`8666`, :pull:`8668`)

xarray/conventions.py

@@ -16,7 +16,7 @@
 )
 from xarray.core.pycompat import is_duck_dask_array
 from xarray.core.utils import emit_user_level_warning
-from xarray.core.variable import Variable
+from xarray.core.variable import IndexVariable, Variable
 
 CF_RELATED_DATA = (
     "bounds",
@@ -84,13 +84,17 @@ def _infer_dtype(array, name=None):
 
 
 def ensure_not_multiindex(var: Variable, name: T_Name = None) -> None:
+    # only the pandas multi-index dimension coordinate cannot be serialized (tuple values)
     if isinstance(var._data, indexing.PandasMultiIndexingAdapter):
-        raise NotImplementedError(
-            f"variable {name!r} is a MultiIndex, which cannot yet be "
-            "serialized. Instead, either use reset_index() "
-            "to convert MultiIndex levels into coordinate variables instead "
-            "or use https://cf-xarray.readthedocs.io/en/latest/coding.html."
-        )
+        if name is None and isinstance(var, IndexVariable):
+            name = var.name
+        if var.dims == (name,):
+            raise NotImplementedError(
+                f"variable {name!r} is a MultiIndex, which cannot yet be "
+                "serialized. Instead, either use reset_index() "
+                "to convert MultiIndex levels into coordinate variables instead "
+                "or use https://cf-xarray.readthedocs.io/en/latest/coding.html."
+            )
 
 
 def _copy_with_dtype(data, dtype: np.typing.DTypeLike):

xarray/tests/test_backends.py

@@ -1246,6 +1246,11 @@ def test_multiindex_not_implemented(self) -> None:
             with self.roundtrip(ds):
                 pass
 
+        # regression GH8628 (can serialize reset multi-index level coordinates)
+        ds_reset = ds.reset_index("x")
+        with self.roundtrip(ds_reset) as actual:
+            assert_identical(actual, ds_reset)
+
 
 class NetCDFBase(CFEncodedBase):
     """Tests for all netCDF3 and netCDF4 backends."""