BUG: fix Index.reindex to preserve name when target is list/ndarray

doc/source/v0.15.0.txt

@@ -1012,3 +1012,4 @@ Bug Fixes
 - Bug in masked series assignment where mismatching types would break alignment (:issue:`8387`)
 - Bug in NDFrame.equals gives false negatives with dtype=object (:issue:`8437`)
 - Bug in assignment with indexer where type diversity would break alignment (:issue:`8258`)
+- Bug in ``NDFrame.loc`` indexing when row/column names were lost when target was a list/ndarray (:issue:`6552`)

pandas/core/frame.py

@@ -2260,17 +2260,15 @@ def _reindex_axes(self, axes, level, limit, method, fill_value, copy):
     def _reindex_index(self, new_index, method, copy, level, fill_value=NA,
                        limit=None):
         new_index, indexer = self.index.reindex(new_index, method, level,
-                                                limit=limit,
-                                                copy_if_needed=True)
+                                                limit=limit)
         return self._reindex_with_indexers({0: [new_index, indexer]},
                                            copy=copy, fill_value=fill_value,
                                            allow_dups=False)
 
     def _reindex_columns(self, new_columns, copy, level, fill_value=NA,
                          limit=None):
         new_columns, indexer = self.columns.reindex(new_columns, level=level,
-                                                    limit=limit,
-                                                    copy_if_needed=True)
+                                                    limit=limit)
         return self._reindex_with_indexers({1: [new_columns, indexer]},
                                            copy=copy, fill_value=fill_value,
                                            allow_dups=False)

pandas/core/generic.py

@@ -1776,8 +1776,8 @@ def reindex_axis(self, labels, axis=0, method=None, level=None, copy=True,
         axis_name = self._get_axis_name(axis)
         axis_values = self._get_axis(axis_name)
         method = com._clean_fill_method(method)
-        new_index, indexer = axis_values.reindex(
-            labels, method, level, limit=limit, copy_if_needed=True)
+        new_index, indexer = axis_values.reindex(labels, method, level,
+                                                 limit=limit)
         return self._reindex_with_indexers(
             {axis: [new_index, indexer]}, method=method, fill_value=fill_value,
             limit=limit, copy=copy)

pandas/core/index.py

@@ -1578,34 +1578,31 @@ def _get_method(self, method):
         }
         return aliases.get(method, method)
 
-    def reindex(self, target, method=None, level=None, limit=None,
-                copy_if_needed=False):
+    def reindex(self, target, method=None, level=None, limit=None):
         """
-        For Index, simply returns the new index and the results of
-        get_indexer. Provided here to enable an interface that is amenable for
-        subclasses of Index whose internals are different (like MultiIndex)
+        Create index with target's values (move/add/delete values as necessary)
 
         Returns
         -------
-        (new_index, indexer, mask) : tuple
+        new_index : pd.Index
+            Resulting index
+        indexer : np.ndarray or None
+            Indices of output values in original index
+
         """
+        # GH6552: preserve names when reindexing to non-named target
+        # (i.e. neither Index nor Series).
+        preserve_names = not hasattr(target, 'name')
+
         target = _ensure_index(target)
         if level is not None:
             if method is not None:
                 raise TypeError('Fill method not supported if level passed')
             _, indexer, _ = self._join_level(target, level, how='right',
                                              return_indexers=True)
         else:
-
             if self.equals(target):
                 indexer = None
-
-                # to avoid aliasing an existing index
-                if (copy_if_needed and target.name != self.name and
-                        self.name is not None):
-                    if target.name is None:
-                        target = self.copy()
-
             else:
                 if self.is_unique:
                     indexer = self.get_indexer(target, method=method,
@@ -1616,6 +1613,10 @@ def reindex(self, target, method=None, level=None, limit=None,
                                          "with a method or limit")
                     indexer, missing = self.get_indexer_non_unique(target)
 
+        if preserve_names and target.nlevels == 1 and target.name != self.name:
+            target = target.copy()
+            target.name = self.name
+
         return target, indexer
 
     def join(self, other, how='left', level=None, return_indexers=False):
@@ -3686,17 +3687,21 @@ def get_indexer(self, target, method=None, limit=None):
 
         return com._ensure_platform_int(indexer)
 
-    def reindex(self, target, method=None, level=None, limit=None,
-                copy_if_needed=False):
+    def reindex(self, target, method=None, level=None, limit=None):
         """
-        Performs any necessary conversion on the input index and calls
-        get_indexer. This method is here so MultiIndex and an Index of
-        like-labeled tuples can play nice together
+        Create index with target's values (move/add/delete values as necessary)
 
         Returns
         -------
-        (new_index, indexer, mask) : (MultiIndex, ndarray, ndarray)
+        new_index : pd.MultiIndex
+            Resulting index
+        indexer : np.ndarray or None
+            Indices of output values in original index
+
         """
+        # GH6552: preserve names when reindexing to non-named target
+        # (i.e. neither Index nor Series).
+        preserve_names = not hasattr(target, 'names')
 
         if level is not None:
             if method is not None:
@@ -3724,6 +3729,11 @@ def reindex(self, target, method=None, level=None, limit=None,
                 # hopefully?
                 target = MultiIndex.from_tuples(target)
 
+        if (preserve_names and target.nlevels == self.nlevels and
+            target.names != self.names):
+            target = target.copy(deep=False)
+            target.names = self.names
+
         return target, indexer
 
     @cache_readonly

pandas/core/internals.py

@@ -3088,7 +3088,7 @@ def reindex_axis(self, new_index, axis, method=None, limit=None,
         """
         new_index = _ensure_index(new_index)
         new_index, indexer = self.axes[axis].reindex(
-            new_index, method=method, limit=limit, copy_if_needed=True)
+            new_index, method=method, limit=limit)
 
         return self.reindex_indexer(new_index, indexer, axis=axis,
                                     fill_value=fill_value, copy=copy)

pandas/tests/test_index.py

@@ -1019,6 +1019,36 @@ def test_nan_first_take_datetime(self):
         exp = Index([idx[-1], idx[0], idx[1]])
         tm.assert_index_equal(res, exp)
 
+    def test_reindex_preserves_name_if_target_is_list_or_ndarray(self):
+        # GH6552
+        idx = pd.Index([0, 1, 2])
+
+        dt_idx = pd.date_range('20130101', periods=3)
+
+        idx.name = None
+        self.assertEqual(idx.reindex([])[0].name, None)
+        self.assertEqual(idx.reindex(np.array([]))[0].name, None)
+        self.assertEqual(idx.reindex(idx.tolist())[0].name, None)
+        self.assertEqual(idx.reindex(idx.tolist()[:-1])[0].name, None)
+        self.assertEqual(idx.reindex(idx.values)[0].name, None)
+        self.assertEqual(idx.reindex(idx.values[:-1])[0].name, None)
+
+        # Must preserve name even if dtype changes.
+        self.assertEqual(idx.reindex(dt_idx.values)[0].name, None)
+        self.assertEqual(idx.reindex(dt_idx.tolist())[0].name, None)
+
+        idx.name = 'foobar'
+        self.assertEqual(idx.reindex([])[0].name, 'foobar')
+        self.assertEqual(idx.reindex(np.array([]))[0].name, 'foobar')
+        self.assertEqual(idx.reindex(idx.tolist())[0].name, 'foobar')
+        self.assertEqual(idx.reindex(idx.tolist()[:-1])[0].name, 'foobar')
+        self.assertEqual(idx.reindex(idx.values)[0].name, 'foobar')
+        self.assertEqual(idx.reindex(idx.values[:-1])[0].name, 'foobar')
+
+        # Must preserve name even if dtype changes.
+        self.assertEqual(idx.reindex(dt_idx.values)[0].name, 'foobar')
+        self.assertEqual(idx.reindex(dt_idx.tolist())[0].name, 'foobar')
+
 
 class Numeric(Base):
 
@@ -3267,6 +3297,30 @@ def test_isin_level_kwarg(self):
 
         self.assertRaises(KeyError, idx.isin, vals_1, level='C')
 
+    def test_reindex_preserves_names_when_target_is_list_or_ndarray(self):
+        # GH6552
+        idx = self.index.copy()
+        target = idx.copy()
+        idx.names = target.names = [None, None]
+
+        other_dtype = pd.MultiIndex.from_product([[1, 2], [3, 4]])
+
+        # list & ndarray cases
+        self.assertEqual(idx.reindex([])[0].names, [None, None])
+        self.assertEqual(idx.reindex(np.array([]))[0].names, [None, None])
+        self.assertEqual(idx.reindex(target.tolist())[0].names, [None, None])
+        self.assertEqual(idx.reindex(target.values)[0].names, [None, None])
+        self.assertEqual(idx.reindex(other_dtype.tolist())[0].names, [None, None])
+        self.assertEqual(idx.reindex(other_dtype.values)[0].names, [None, None])
+
+        idx.names = ['foo', 'bar']
+        self.assertEqual(idx.reindex([])[0].names, ['foo', 'bar'])
+        self.assertEqual(idx.reindex(np.array([]))[0].names, ['foo', 'bar'])
+        self.assertEqual(idx.reindex(target.tolist())[0].names, ['foo', 'bar'])
+        self.assertEqual(idx.reindex(target.values)[0].names, ['foo', 'bar'])
+        self.assertEqual(idx.reindex(other_dtype.tolist())[0].names, ['foo', 'bar'])
+        self.assertEqual(idx.reindex(other_dtype.values)[0].names, ['foo', 'bar'])
+
 
 def test_get_combined_index():
     from pandas.core.index import _get_combined_index

pandas/tests/test_indexing.py

@@ -3836,17 +3836,14 @@ def test_iloc_empty_list_indexer_is_ok(self):
         assert_frame_equal(df.iloc[[],:], df.iloc[:0, :])  # horizontal empty
         assert_frame_equal(df.iloc[[]], df.iloc[:0, :])  # horizontal empty
 
-    # FIXME: fix loc & xs
     def test_loc_empty_list_indexer_is_ok(self):
-        raise nose.SkipTest('loc discards columns names')
         from pandas.util.testing import makeCustomDataframe as mkdf
         df = mkdf(5, 2)
         assert_frame_equal(df.loc[:,[]], df.iloc[:, :0])  # vertical empty
         assert_frame_equal(df.loc[[],:], df.iloc[:0, :])  # horizontal empty
         assert_frame_equal(df.loc[[]], df.iloc[:0, :])  # horizontal empty
 
     def test_ix_empty_list_indexer_is_ok(self):
-        raise nose.SkipTest('ix discards columns names')
         from pandas.util.testing import makeCustomDataframe as mkdf
         df = mkdf(5, 2)
         assert_frame_equal(df.ix[:,[]], df.iloc[:, :0])  # vertical empty