ENH: allow index to be referenced by name

closes #8162, #10816

Author: hsharrison

doc/source/whatsnew/v0.18.0.txt

@@ -432,6 +432,7 @@ Other enhancements
 - Added Google ``BigQuery`` service account authentication support, which enables authentication on remote servers. (:issue:`11881`). For further details see :ref:`here <io.bigquery_authentication>`
 - ``HDFStore`` is now iterable: ``for k in store`` is equivalent to ``for k in store.keys()`` (:issue:`12221`).
 - The entire codebase has been ``PEP``-ified (:issue:`12096`)
+- Index (or index levels, with a MultiIndex) can now be referenced like column names (:issue:`8162`, :issue:`10816`).
 
 .. _whatsnew_0180.api_breaking:
 

pandas/core/frame.py

@@ -2018,7 +2018,30 @@ def _getitem_array(self, key):
             indexer = key.nonzero()[0]
             return self.take(indexer, axis=0, convert=False)
         else:
-            indexer = self.ix._convert_to_indexer(key, axis=1)
+            try:
+                indexer = self.ix._convert_to_indexer(key, axis=1)
+
+            except KeyError:
+                if self.index.name in key:
+                    ix_name = self.index.name
+                    ix_ix = key.index(ix_name)
+
+                elif (isinstance(self.index, MultiIndex) and
+                      any(item in self.index.names for item in key)):
+                    for item in key:
+                        if item in self.index.names:
+                            ix_name = item
+                            ix_ix = key.index(item)
+
+                else:
+                    raise
+
+                key.remove(ix_name)
+                ix_col = self[ix_name]
+                other_cols = self[key]
+                other_cols.insert(ix_ix, ix_name, ix_col)
+                return other_cols
+
             return self.take(indexer, axis=1, convert=True)
 
     def _getitem_multilevel(self, key):

pandas/core/generic.py

@@ -1306,8 +1306,20 @@ def _get_item_cache(self, item):
         cache = self._item_cache
         res = cache.get(item)
         if res is None:
-            values = self._data.get(item)
-            res = self._box_item_values(item, values)
+            try:
+                values = self._data.get(item)
+                res = self._box_item_values(item, values)
+            except KeyError:
+                if hasattr(self, 'index') and self.index.name == item:
+                    res = self.index.to_series()
+
+                elif (isinstance(self.index, MultiIndex) and
+                      item in self.index.names):
+                    res = pd.Series(self.index.get_level_values(item).values,
+                                    index=self.index, name=item)
+
+                else:
+                    raise
             cache[item] = res
             res._set_as_cached(item, self)
 
@@ -2623,10 +2635,13 @@ def __getattr__(self, name):
         if (name in self._internal_names_set or name in self._metadata or
                 name in self._accessors):
             return object.__getattribute__(self, name)
-        else:
-            if name in self._info_axis:
+        elif (
+            name in self._info_axis or
+            name == self.index.name or
+            (isinstance(self.index, MultiIndex) and name in self.index.names)
+        ):
                 return self[name]
-            return object.__getattribute__(self, name)
+        return object.__getattribute__(self, name)
 
     def __setattr__(self, name, value):
         """After regular attribute access, try setting the name

pandas/tests/test_generic.py

@@ -1853,6 +1853,81 @@ def test_to_xarray(self):
                            expected,
                            check_index_type=False)
 
+    def test_getitem_index(self):
+        # GH8162
+        idx = pd.Index(list('abc'), name='idx')
+        df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]}, index=idx)
+        expected = pd.Series(['a', 'b', 'c'], index=idx, name='idx')
+
+        assert_series_equal(df['idx'], expected)
+        assert_series_equal(df.idx, expected)
+
+    def test_getitem_index_listlike(self):
+        idx = pd.Index(list('abc'), name='idx')
+        df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]}, index=idx)
+        assert_frame_equal(
+            df[['idx', 'B']],
+            pd.DataFrame([
+                ['a', 4],
+                ['b', 5],
+                ['c', 6],
+            ],
+                columns=['idx', 'B'],
+                index=idx)
+        )
+        assert_frame_equal(
+            df[['idx', 'A', 'B']],
+            pd.DataFrame([
+                ['a', 1, 4],
+                ['b', 2, 5],
+                ['c', 3, 6],
+            ],
+                columns=['idx', 'A', 'B'],
+                index=idx)
+        )
+
+    def test_getitem_multiindex_level(self):
+        # GH10816
+        idx = pd.MultiIndex.from_product([list('abc'), list('fg')],
+                                         names=['lev0', 'lev1'])
+        df = pd.DataFrame({'A': range(6), 'B': range(10, 16)}, index=idx)
+        expected = pd.Series(list('aabbcc'), index=idx, name='lev0')
+
+        assert_series_equal(df['lev0'], expected)
+        assert_series_equal(df.lev0, expected)
+
+    def test_getitem_multiindex_level_listlike(self):
+        idx = pd.MultiIndex.from_product([list('abc'), list('fg')],
+                                         names=['lev0', 'lev1'])
+        df = pd.DataFrame({'A': range(6), 'B': range(10, 16)}, index=idx)
+        assert_frame_equal(
+            df[['A', 'lev1']],
+            pd.DataFrame([
+                [0, 'f'],
+                [1, 'g'],
+                [2, 'f'],
+                [3, 'g'],
+                [4, 'f'],
+                [5, 'g'],
+            ],
+                columns=['A', 'lev1'],
+                index=idx)
+        )
+
+        assert_frame_equal(
+            df[['A', 'B', 'lev1', 'lev0']],
+            pd.DataFrame([
+                [0, 10, 'f', 'a'],
+                [1, 11, 'g', 'a'],
+                [2, 12, 'f', 'b'],
+                [3, 13, 'g', 'b'],
+                [4, 14, 'f', 'c'],
+                [5, 15, 'g', 'c'],
+            ],
+                columns=['A', 'B', 'lev1', 'lev0'],
+                index=idx)
+        )
+
 
 class TestPanel(tm.TestCase, Generic):
     _typ = Panel