BUG: .str methods with expand=True may raise ValueError if input has name

doc/source/whatsnew/v0.18.1.txt

@@ -225,6 +225,7 @@ Bug Fixes
 
 
 
+- Bug in ``.str`` accessor methods may raise ``ValueError`` if input has ``name`` and the result is ``DataFrame`` or ``MultiIndex`` (:issue:`12617`)
 
 
 - Bug in ``CategoricalIndex.get_loc`` returns different result from regular ``Index`` (:issue:`12531`)

pandas/core/strings.py

@@ -1329,12 +1329,15 @@ def cons_row(x):
         if not isinstance(expand, bool):
             raise ValueError("expand must be True or False")
 
-        if name is None:
-            name = getattr(result, 'name', None)
-        if name is None:
-            # do not use logical or, _orig may be a DataFrame
-            # which has "name" column
-            name = self._orig.name
+        if expand is False:
+            # if expand is False, result should have the same name
+            # as the original otherwise specified
+            if name is None:
+                name = getattr(result, 'name', None)
+            if name is None:
+                # do not use logical or, _orig may be a DataFrame
+                # which has "name" column
+                name = self._orig.name
 
         # Wait until we are sure result is a Series or Index before
         # checking attributes (GH 12180)

pandas/tests/test_strings.py

@@ -1938,6 +1938,30 @@ def test_rsplit_to_multiindex_expand(self):
         tm.assert_index_equal(result, exp)
         self.assertEqual(result.nlevels, 2)
 
+    def test_split_with_name(self):
+        # GH 12617
+
+        # should preserve name
+        s = Series(['a,b', 'c,d'], name='xxx')
+        res = s.str.split(',')
+        exp = Series([('a', 'b'), ('c', 'd')], name='xxx')
+        tm.assert_series_equal(res, exp)
+
+        res = s.str.split(',', expand=True)
+        exp = DataFrame([['a', 'b'], ['c', 'd']])
+        tm.assert_frame_equal(res, exp)
+
+        idx = Index(['a,b', 'c,d'], name='xxx')
+        res = idx.str.split(',')
+        exp = Index([['a', 'b'], ['c', 'd']], name='xxx')
+        self.assertTrue(res.nlevels, 1)
+        tm.assert_index_equal(res, exp)
+
+        res = idx.str.split(',', expand=True)
+        exp = MultiIndex.from_tuples([('a', 'b'), ('c', 'd')])
+        self.assertTrue(res.nlevels, 2)
+        tm.assert_index_equal(res, exp)
+
     def test_partition_series(self):
         values = Series(['a_b_c', 'c_d_e', NA, 'f_g_h'])
 
@@ -2059,6 +2083,31 @@ def test_partition_to_dataframe(self):
                          2: ['c', 'e', np.nan, 'h']})
         tm.assert_frame_equal(result, exp)
 
+    def test_partition_with_name(self):
+        # GH 12617
+
+        s = Series(['a,b', 'c,d'], name='xxx')
+        res = s.str.partition(',')
+        exp = DataFrame({0: ['a', 'c'], 1: [',', ','], 2: ['b', 'd']})
+        tm.assert_frame_equal(res, exp)
+
+        # should preserve name
+        res = s.str.partition(',', expand=False)
+        exp = Series([('a', ',', 'b'), ('c', ',', 'd')], name='xxx')
+        tm.assert_series_equal(res, exp)
+
+        idx = Index(['a,b', 'c,d'], name='xxx')
+        res = idx.str.partition(',')
+        exp = MultiIndex.from_tuples([('a', ',', 'b'), ('c', ',', 'd')])
+        self.assertTrue(res.nlevels, 3)
+        tm.assert_index_equal(res, exp)
+
+        # should preserve name
+        res = idx.str.partition(',', expand=False)
+        exp = Index(np.array([('a', ',', 'b'), ('c', ',', 'd')]), name='xxx')
+        self.assertTrue(res.nlevels, 1)
+        tm.assert_index_equal(res, exp)
+
     def test_pipe_failures(self):
         # #2119
         s = Series(['A|B|C'])