fix safe_cast_to_index (#3001)

doc/whats-new.rst

@@ -70,8 +70,10 @@ Bug fixes
   By `Deepak Cherian <https://github.com/dcherian`_.
 - A deep copy deep-copies the coords (:issue:`1463`)
   By `Martin Pletcher <https://github.com/pletchm>`_.
-- Removed usages of `pytest.config`, which is deprecated (:issue:`2988`:)
+- Removed usages of `pytest.config`, which is deprecated (:issue:`2988`)
   By `Maximilian Roos <https://github.com/max-sixty>`_.
+- Fixed performance issues with cftime installed (:issue:`3000`)
+  By `0x0L <https://github.com/0x0L>`_.
 
 .. _whats-new.0.12.1:
 

xarray/coding/cftimeindex.py

@@ -184,7 +184,7 @@ def get_date_type(self):
 def assert_all_valid_date_type(data):
     import cftime
 
-    if data.size:
+    if len(data) > 0:
         sample = data[0]
         date_type = type(sample)
         if not isinstance(sample, cftime.datetime):
@@ -229,12 +229,12 @@ class CFTimeIndex(pd.Index):
     date_type = property(get_date_type)
 
     def __new__(cls, data, name=None):
+        assert_all_valid_date_type(data)
         if name is None and hasattr(data, 'name'):
             name = data.name
 
         result = object.__new__(cls)
         result._data = np.array(data, dtype='O')
-        assert_all_valid_date_type(result._data)
         result.name = name
         return result
 

xarray/core/utils.py

@@ -62,7 +62,7 @@ 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:
             return CFTimeIndex(index)
         except (ImportError, TypeError):

xarray/tests/test_dataarray.py

@@ -1760,6 +1760,12 @@ 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, :1].stack(dim=('x', 'y')).dim.to_index()
+        b = pd.MultiIndex(levels=[pd.Int64Index([]), pd.Int64Index([0])],
+                          labels=[[], []], names=['x', 'y'])
+        pd.util.testing.assert_index_equal(a, b)
+
         actual = orig.stack(z=['x', 'y']).unstack('z').drop(['x', 'y'])
         assert_identical(orig, actual)