Fix for groupby math with virtual coordinates

Author: shoyer

xray/core/dataset.py

@@ -415,21 +415,6 @@ def __getstate__(self):
         state['_file_obj'] = None
         return state
 
-    # @property
-    # def arrays(self):
-    #     """Dictionary of Variable objects contained in this dataset.
-
-    #     This is a low-level interface into the contents of a dataset. The
-    #     dictionary is frozen to prevent it from being modified in ways that
-    #     could create an inconsistent dataset (e.g., by setting variables with
-    #     inconsistent dimensions).
-
-    #     In general, to access and modify dataset contents, you should use
-    #     dictionary methods on the dataset itself instead of the variables
-    #     dictionary.
-    #     """
-    #     return Frozen(self._arrays)
-
     @property
     def variables(self):
         """Deprecated; do not use"""
@@ -1436,7 +1421,7 @@ def differs(vname, v):
 
         if not isinstance(dim, basestring):
             # add dimension last to ensure that its in the final Dataset
-            concatenated[dim_name] = dim
+            concatenated.coords[dim_name] = dim
 
         return concatenated
 

xray/test/test_dataset.py

@@ -883,6 +883,15 @@ def test_groupby_math(self):
         with self.assertRaisesRegexp(TypeError, 'only support arithmetic'):
             grouped + grouped
 
+    def test_groupby_math_virtual(self):
+        ds = Dataset({'x': ('t', [1, 2, 3])},
+                     {'t': pd.date_range('20100101', periods=3)})
+        grouped = ds.groupby('t.day')
+        actual = grouped - grouped.mean()
+        expected = Dataset({'x': ('t', [0, 0, 0])},
+                           {'t': ds['t'], 't.day': ds['t.day']})
+        self.assertDatasetIdentical(actual, expected)
+
     def test_concat(self):
         data = create_test_data()