Bug in DecodedCFDatetimeArray (#1112)

* Bug in DecodedCFDatetimeArray

* simpler unittest

* mini nit

Author: fmaussion

xarray/conventions.py

@@ -382,8 +382,8 @@ def __init__(self, array, units, calendar=None):
         # Verify that at least the first and last date can be decoded
         # successfully. Otherwise, tracebacks end up swallowed by
         # Dataset.__repr__ when users try to view their lazily decoded array.
-        example_value = np.concatenate([first_n_items(array, 1),
-                                        last_item(array), [0]])
+        example_value = np.concatenate([first_n_items(array, 1) or [0],
+                                        last_item(array) or [0]])
 
         try:
             result = decode_cf_datetime(example_value, units, calendar)

xarray/test/test_conventions.py

@@ -494,6 +494,12 @@ def test_dataset_repr_with_netcdf4_datetimes(self):
         ds = decode_cf(Dataset({'time': ('time', [0, 1], attrs)}))
         self.assertIn('(time) datetime64[ns]', repr(ds))
 
+        # this should not throw a warning (GH1111)
+        with warnings.catch_warnings():
+            warnings.filterwarnings('error')
+            conventions.DecodedCFDatetimeArray(np.asarray([722624]),
+                                               "days since 0001-01-01")
+
 
 class TestNativeEndiannessArray(TestCase):
     def test(self):