fix safe_cast_to_index

doc/whats-new.rst

@@ -63,6 +63,9 @@ Bug fixes
   By `Martin Pletcher <https://github.com/pletchm>`_.
 - Removed usages of `pytest.config`, which is deprecated (:issue:`2988`:)
   By `Maximilian Roos <https://github.com/max-sixty>`_.
+- Fixed a bug in `utils.safe_cast_to_index` and performance issues when
+  `cftime` is installed (:issue:`3000`:)
+  By `0x0L <https://github.com/0x0L>`_.
 
 .. _whats-new.0.12.1:
 

xarray/core/utils.py

@@ -62,8 +62,11 @@ def wrapper(*args, **kwargs):
 def _maybe_cast_to_cftimeindex(index: pd.Index) -> pd.Index:
     from ..coding.cftimeindex import CFTimeIndex
 
-    if index.dtype == 'O':
+    if len(index) > 0 and index.dtype == 'O':
         try:
+            import cftime
+            if not isinstance(index[0], cftime.datetime):
+                return index
             return CFTimeIndex(index)
         except (ImportError, TypeError):
             return index

xarray/tests/test_dataarray.py

@@ -1760,6 +1760,11 @@ def test_stack_unstack(self):
         orig = DataArray([[0, 1], [2, 3]], dims=['x', 'y'], attrs={'foo': 2})
         assert_identical(orig, orig.unstack())
 
+        # test GH3000
+        a = orig[:0].stack(dim=('x', 'y')).dim.to_index()
+        b = pd.MultiIndex(levels=[[], [0, 1]], codes=[[], []], names=['x', 'y'])
+        assert a.equals(b)
+
         actual = orig.stack(z=['x', 'y']).unstack('z').drop(['x', 'y'])
         assert_identical(orig, actual)