Ensure Dataset.dims wraps a normal dict, not an OrderedDict (#923)

* Ensure Dataset.dims wraps a normal dict, not an OrderedDict

This was introduced by GH857.

* Alternate fix

Author: shoyer

xarray/core/coordinates.py

@@ -164,7 +164,7 @@ def _update_coords(self, coords):
 
         self._data._variables = variables
         self._data._coord_names.update(updated_coord_names)
-        self._data._dims = dims
+        self._data._dims = dict(dims)
 
     def __delitem__(self, key):
         if key in self:

xarray/core/dataset.py

@@ -351,7 +351,7 @@ def _construct_direct(cls, variables, coord_names, dims=None, attrs=None,
 
     @classmethod
     def _from_vars_and_coord_names(cls, variables, coord_names):
-        dims = calculate_dimensions(variables)
+        dims = dict(calculate_dimensions(variables))
         return cls._construct_direct(variables, coord_names, dims)
 
     def _replace_vars_and_dims(self, variables, coord_names=None, dims=None,

xarray/core/merge.py

@@ -395,7 +395,7 @@ def merge_core(objs, compat='broadcast_equals', join='outer', priority_arg=None,
                          'coordinates or not in the merged result: %s'
                          % ambiguous_coords)
 
-    return variables, coord_names, dims
+    return variables, coord_names, dict(dims)
 
 
 def merge(objects, compat='broadcast_equals', join='outer'):

xarray/test/test_dataset.py

@@ -297,6 +297,13 @@ def test_properties(self):
         ds = create_test_data()
         self.assertEqual(ds.dims,
                          {'dim1': 8, 'dim2': 9, 'dim3': 10, 'time': 20})
+        self.assertEqual(list(ds.dims), sorted(ds.dims))
+
+        # These exact types aren't public API, but this makes sure we don't
+        # change them inadvertently:
+        self.assertIsInstance(ds.dims, utils.Frozen)
+        self.assertIsInstance(ds.dims.mapping, utils.SortedKeysDict)
+        self.assertIs(type(ds.dims.mapping.mapping), dict)
 
         self.assertItemsEqual(ds, list(ds.variables))
         self.assertItemsEqual(ds.keys(), list(ds.variables))