From 97a829a5ae5d849a5c20ec019f31f6879841b843 Mon Sep 17 00:00:00 2001
From: Stephan Hoyer <shoyer@gmail.com>
Date: Sat, 17 Sep 2016 11:50:55 -0700
Subject: [PATCH] MultiIndex level coordinates as Dataset attributes

Follow-up on GH947

This ensures that you can pull out MultiIndex levels via attribute style
access on Dataset objects, as well as DataArray objects.

CC benbovy
---
 xarray/core/common.py       | 2 +-
 xarray/core/coordinates.py  | 6 +++---
 xarray/core/dataarray.py    | 4 ++--
 xarray/core/dataset.py      | 7 ++++++-
 xarray/test/test_dataset.py | 3 +++
 5 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/xarray/core/common.py b/xarray/core/common.py
index 5b04f45fc8e..831fb89d125 100644
--- a/xarray/core/common.py
+++ b/xarray/core/common.py
@@ -181,7 +181,7 @@ class AttrAccessMixin(object):
     @property
     def _attr_sources(self):
         """List of places to look-up items for attribute-style access"""
-        return [self, self.attrs]
+        return []
 
     def __getattr__(self, name):
         if name != '__setstate__':
diff --git a/xarray/core/coordinates.py b/xarray/core/coordinates.py
index 30240c16b27..9eca9009460 100644
--- a/xarray/core/coordinates.py
+++ b/xarray/core/coordinates.py
@@ -215,8 +215,8 @@ def __delitem__(self, key):
         del self._data._coords[key]
 
 
-class DataArrayLevelCoordinates(AbstractCoordinates):
-    """Dictionary like container for DataArray MultiIndex level coordinates.
+class LevelCoordinates(AbstractCoordinates):
+    """Dictionary like container for MultiIndex level coordinates.
 
     Used for attribute style lookup. Not returned directly by any
     public methods.
@@ -232,7 +232,7 @@ def _names(self):
     def variables(self):
         level_coords = OrderedDict(
             (k, self._data[v].variable.get_level_variable(k))
-             for k, v in self._data._level_coords.items())
+            for k, v in self._data._level_coords.items())
         return Frozen(level_coords)
 
 
diff --git a/xarray/core/dataarray.py b/xarray/core/dataarray.py
index 41bc17f7e07..5836167326e 100644
--- a/xarray/core/dataarray.py
+++ b/xarray/core/dataarray.py
@@ -14,7 +14,7 @@
 from . import utils
 from .alignment import align
 from .common import AbstractArray, BaseDataObject, squeeze
-from .coordinates import (DataArrayCoordinates, DataArrayLevelCoordinates,
+from .coordinates import (DataArrayCoordinates, LevelCoordinates,
                           Indexes)
 from .dataset import Dataset
 from .pycompat import iteritems, basestring, OrderedDict, zip
@@ -467,7 +467,7 @@ def __delitem__(self, key):
     @property
     def _attr_sources(self):
         """List of places to look-up items for attribute-style access"""
-        return [self.coords, DataArrayLevelCoordinates(self), self.attrs]
+        return [self.coords, LevelCoordinates(self), self.attrs]
 
     def __contains__(self, key):
         return key in self._coords
diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 65353fc51ef..e0833ceb8d5 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -15,7 +15,7 @@
 from . import formatting
 from .. import conventions
 from .alignment import align
-from .coordinates import DatasetCoordinates, Indexes
+from .coordinates import DatasetCoordinates, LevelCoordinates, Indexes
 from .common import ImplementsDatasetReduce, BaseDataObject
 from .merge import (dataset_update_method, dataset_merge_method,
                     merge_data_and_coords)
@@ -487,6 +487,11 @@ def __deepcopy__(self, memo=None):
         # copy.deepcopy
         return self.copy(deep=True)
 
+    @property
+    def _attr_sources(self):
+        """List of places to look-up items for attribute-style access"""
+        return [self, LevelCoordinates(self), self.attrs]
+
     def __contains__(self, key):
         """The 'in' operator will return true or false depending on whether
         'key' is an array in the dataset or not.
diff --git a/xarray/test/test_dataset.py b/xarray/test/test_dataset.py
index a1da10b4ca5..02f1fbf7171 100644
--- a/xarray/test/test_dataset.py
+++ b/xarray/test/test_dataset.py
@@ -1540,6 +1540,9 @@ def test_virtual_variable_multiindex(self):
                              name='hour', coords=[mindex], dims='x')
         self.assertDataArrayIdentical(expected, data['level_date.hour'])
 
+        # attribute style access
+        self.assertDataArrayIdentical(data.level_str, data['level_str'])
+
     def test_time_season(self):
         ds = Dataset({'t': pd.date_range('2000-01-01', periods=12, freq='M')})
         expected = ['DJF'] * 2 + ['MAM'] * 3 + ['JJA'] * 3 + ['SON'] * 3 + ['DJF']