Disable setting DataArray.dims inplace

The existing behavior was undocumented and non-obvious -- it's not clear
what happens with the associated coordinates (i.e., should it be more
like rename or swap_dims?).

Author: shoyer

xray/core/dataarray.py

@@ -309,14 +309,8 @@ def dims(self):
 
     @dims.setter
     def dims(self, value):
-        with self._set_new_dataset() as ds:
-            if not len(value) == self.ndim:
-                raise ValueError('%s dimensions supplied but data has ndim=%s'
-                                 % (len(value), self.ndim))
-            name_map = dict(zip(self.dims, value))
-            ds.rename(name_map, inplace=True)
-        if self.name in name_map:
-            self._name = name_map[self.name]
+        raise AttributeError('you cannot assign dims on a DataArray. Use '
+                             '.rename() or .swap_dims() instead.')
 
     def _item_key_to_dict(self, key):
         if utils.is_dict_like(key):

xray/test/test_dataarray.py

@@ -69,13 +69,8 @@ def test_dims(self):
         arr = self.dv
         self.assertEqual(arr.dims, ('x', 'y'))
 
-        arr.dims = ('w', 'z')
-        self.assertEqual(arr.dims, ('w', 'z'))
-
-        x = Dataset({'x': ('x', np.arange(5))})['x']
-        x.dims = ('y',)
-        self.assertEqual(x.dims, ('y',))
-        self.assertEqual(x.name, 'y')
+        with self.assertRaisesRegexp(AttributeError, 'you cannot assign'):
+            arr.dims = ('w', 'z')
 
     def test_encoding(self):
         expected = {'foo': 'bar'}