BUG: Fixes GH3215 (#3220)

* BUG: Fixes GH3215

Explicit cast to numpy array to avoid np.ravel calling out to dask

* Added test

* Added assertion to test

* Removed assertion. Didn't work

* PEP8 and flake fixes

* Ran black

* Implement suggested fixes

* Added assert test

* Added valid assert_identical test

* add requires_dask

xarray/core/formatting.py

@@ -96,7 +96,7 @@ def last_item(array):
         return []
 
     indexer = (slice(-1, None),) * array.ndim
-    return np.ravel(array[indexer]).tolist()
+    return np.ravel(np.asarray(array[indexer])).tolist()
 
 
 def format_timestamp(t):

xarray/tests/test_conventions.py

@@ -278,6 +278,26 @@ def test_decode_cf_with_dask(self):
         )
         assert_identical(decoded, conventions.decode_cf(original).compute())
 
+    @requires_dask
+    def test_decode_dask_times(self):
+        original = Dataset.from_dict(
+            {
+                "coords": {},
+                "dims": {"time": 5},
+                "data_vars": {
+                    "average_T1": {
+                        "dims": ("time",),
+                        "attrs": {"units": "days since 1958-01-01 00:00:00"},
+                        "data": [87659.0, 88024.0, 88389.0, 88754.0, 89119.0],
+                    }
+                },
+            }
+        )
+        assert_identical(
+            conventions.decode_cf(original.chunk()),
+            conventions.decode_cf(original).chunk(),
+        )
+
 
 class CFEncodedInMemoryStore(WritableCFDataStore, InMemoryDataStore):
     def encode_variable(self, var):