fix cf_xarray.utils._is_datetime_like (#289)

Co-authored-by: dcherian <[email protected]>

cf_xarray/tests/test_accessor.py

@@ -1409,6 +1409,23 @@ def test_cf_standard_name_table_version():
     assert expected_info == actual_info
 
 
+def test_add_canonical_attributes_0_dim():
+    """test if works for variables with 0 dimension"""
+    xr.DataArray(
+        0, attrs={"standard_name": "sea_water_potential_temperature"}
+    ).cf.add_canonical_attributes()
+
+
+def test_datetime_like():
+    """test for 0 or >= 2 time dimensions"""
+    da = xr.DataArray(
+        np.timedelta64(1, "D"),
+        attrs={"standard_name": "sea_water_age_since_surface_contact"},
+    )
+    new_attrs = da.cf.add_canonical_attributes().attrs
+    assert "units" not in new_attrs and "description" in new_attrs
+
+
 @pytest.mark.parametrize("override", [True, False])
 @pytest.mark.parametrize("skip", ["units", None])
 @pytest.mark.parametrize("verbose", [True, False])

cf_xarray/utils.py

@@ -3,24 +3,42 @@
 from typing import Any, Dict, Iterable
 from xml.etree import ElementTree
 
+import numpy as np
 from xarray import DataArray
 
+try:
+    import cftime
+except ImportError:
+    cftime = None
+
+
+def _contains_cftime_datetimes(array) -> bool:
+    """Check if an array contains cftime.datetime objects"""
+    # Copied / adapted from xarray.core.common
+    from xarray.core.pycompat import is_duck_dask_array
+
+    if cftime is None:
+        return False
+    else:
+        if array.dtype == np.dtype("O") and array.size > 0:
+            sample = array.ravel()[0]
+            if is_duck_dask_array(sample):
+                sample = sample.compute()
+                if isinstance(sample, np.ndarray):
+                    sample = sample.item()
+            return isinstance(sample, cftime.datetime)
+        else:
+            return False
 
-def _is_datetime_like(da: DataArray) -> bool:
-    import numpy as np
 
+def _is_datetime_like(da: DataArray) -> bool:
     if np.issubdtype(da.dtype, np.datetime64) or np.issubdtype(
         da.dtype, np.timedelta64
     ):
         return True
-
-    try:
-        import cftime
-
-        if isinstance(da.data[0], cftime.datetime):
-            return True
-    except ImportError:
-        pass
+    # if cftime was not imported, _contains_cftime_datetimes will return False
+    if _contains_cftime_datetimes(da.data):
+        return True
 
     return False