FIX: interesction and union don't change index names anymore. fixes #9943 partly #9862

Author: hsperr

doc/source/whatsnew/v0.16.1.txt

@@ -243,3 +243,13 @@ Bug Fixes
 
 
 - Bug in hiding ticklabels with subplots and shared axes when adding a new plot to an existing grid of axes (:issue:`9158`)
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+- ``Union`` and ``intersection`` now wont change index name. (:issue:`9943`)

pandas/core/index.py

@@ -1272,7 +1272,7 @@ def union(self, other):
         return self._wrap_union_result(other, result)
 
     def _wrap_union_result(self, other, result):
-        name = self.name if self.name == other.name else None
+        name = self.name#GH 9943, 9862 if self.name == other.name else None
         return self.__class__(data=result, name=name)
 
     def intersection(self, other):
@@ -1319,8 +1319,9 @@ def intersection(self, other):
             indexer = indexer[indexer != -1]
 
         taken = self.take(indexer)
-        if self.name != other.name:
-            taken.name = None
+        #GH 9943, 9862
+        #if self.name != other.name:
+        #    taken.name = None
         return taken
 
     def difference(self, other):

pandas/tests/test_index.py

@@ -17,7 +17,7 @@
                     CategoricalIndex, DatetimeIndex, TimedeltaIndex, PeriodIndex)
 from pandas.core.index import InvalidIndexError, NumericIndex
 from pandas.util.testing import (assert_almost_equal, assertRaisesRegexp,
-                                 assert_copy)
+                                 assert_copy, assert_frame_equal)
 from pandas import compat
 from pandas.compat import long
 
@@ -603,6 +603,15 @@ def test_shift(self):
         shifted.name = 'shifted'
         self.assertEqual(shifted.name, shifted.shift(1, 'D').name)
 
+    def test_intersection_preserves_name(self):
+        #GH 9943
+        df = pd.DataFrame([np.nan, np.nan], columns = ['tags'], index=pd.Int64Index([4815961, 4815962], dtype='int64', name='id'))
+        assert str(df) == '         tags\nid           \n4815961   NaN\n4815962   NaN'
+        L = [4815962]
+        assert list(L) == list(df.index.intersection(L))
+        assert  df.ix[L].tags.index.name == df.ix[df.index.intersection(L)].tags.index.name
+        assert_frame_equal(df.ix[L], df.ix[df.index.intersection(L)])
+
     def test_intersection(self):
         first = self.strIndex[:20]
         second = self.strIndex[:10]
@@ -627,7 +636,7 @@ def test_intersection(self):
 
         # if target name is different, it will be reset
         idx3 = Index([3, 4, 5, 6, 7], name='other')
-        expected3 = Index([3, 4, 5], name=None)
+        expected3 = Index([3, 4, 5], name='idx')
         result3 = idx1.intersection(idx3)
         self.assertTrue(result3.equals(expected3))
         self.assertEqual(result3.name, expected3.name)
@@ -677,9 +686,10 @@ def test_union(self):
         union = first.union(second)
         self.assertEqual(union.name, 'A')
 
+        #preserve name if different GH 9862
         second.name = 'B'
         union = first.union(second)
-        self.assertIsNone(union.name)
+        self.assertEquals('A', union.name)
 
     def test_add(self):