introspect results from map so that if the output array has tuples we create a multiindex instead of an index

Author: nateyoder

doc/source/whatsnew/v0.20.0.txt

@@ -91,6 +91,25 @@ Other enhancements
 Backwards incompatible API changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+-  ``map`` on an ``Index`` now returns an ``Index``, not an array (:issue:`12766`)
+.. ipython:: python
+
+   idx = Index([1, 2])
+   idx
+
+Previous Behavior:
+
+.. code-block:: ipython
+
+   In [3]: idx.map(lambda x: x * 2)
+   Out[3]: array([2, 4])
+
+New Behavior:
+
+.. ipython:: python
+
+    idx.map(lambda x: x * 2)
+
 .. _whatsnew_0200.api:
 
 

pandas/indexes/base.py

@@ -2439,10 +2439,17 @@ def map(self, mapper):
         -------
         applied : Index
             The output of the mapping function applied to the index.
+            If the function returns a tuple a
         """
+        from .multi import MultiIndex
+        mapped_values = self._arrmap(self.values, mapper)
         attributes = self._get_attributes_dict()
+        if mapped_values.size and isinstance(mapped_values[0], tuple):
+            return MultiIndex.from_tuples(mapped_values,
+                                          names=attributes.get('name'))
+
         attributes['copy'] = False
-        return Index(self._arrmap(self.values, mapper), **attributes)
+        return Index(mapped_values, **attributes)
 
     def isin(self, values, level=None):
         """

pandas/tests/indexes/test_base.py

@@ -768,20 +768,24 @@ def test_sub(self):
         self.assertRaises(TypeError, lambda: idx.tolist() - idx)
 
     def test_map_identity_mapping(self):
+        # GH 12766
         for name, cur_index in self.indices.items():
-            if name == 'tuples':
-                expected = Index(cur_index.values, tupleize_cols=False)
-                self.assert_index_equal(expected, cur_index.map(lambda x: x))
-            else:
-                self.assert_index_equal(cur_index, cur_index.map(lambda x: x))
+            self.assert_index_equal(cur_index, cur_index.map(lambda x: x))
 
-    def test_map_that_returns_tuples_creates_index_not_multi_index(self):
+    def test_map_that_returns_tuples_creates_multi_index(self):
+        # GH 12766
         boolean_index = tm.makeIntIndex(3).map(lambda x: (x, x == 1))
-        expected = Index([(0, False), (1, True), (2, False)],
-                         tupleize_cols=False)
+        expected = MultiIndex.from_tuples([(0, False), (1, True), (2, False)])
+        self.assert_index_equal(boolean_index, expected)
+
+    def test_map_that_returns_a_length_one_tuple_creates_an_index(self):
+        # GH 12766
+        boolean_index = tm.makeIntIndex(3).map(lambda x: (x, ))
+        expected = Index([(0, ), (1, ), (2, )])
         self.assert_index_equal(boolean_index, expected)
 
     def test_map_that_reduces_multi_index_to_single_index_returns_index(self):
+        # GH 12766
         first_level = ['foo', 'bar', 'baz']
         multi_index = MultiIndex.from_tuples(lzip(first_level, [1, 2, 3]))
         reduced_index = multi_index.map(lambda x: x[0])

pandas/tests/indexes/test_category.py

@@ -218,6 +218,7 @@ def test_map(self):
                                   ordered=False, name='XXX')
         tm.assert_index_equal(result, exp)
 
+        # GH 12766: Return an index not an array
         tm.assert_index_equal(ci.map(lambda x: 1),
                               Index(np.array([1] * 5, dtype=np.int64), name='XXX'))
 

pandas/tests/test_categorical.py

@@ -1669,6 +1669,7 @@ def test_map(self):
         tm.assert_categorical_equal(result, exp)
 
         result = c.map(lambda x: 1)
+        # GH 12766: Return an index not an array
         tm.assert_index_equal(result, Index(np.array([1] * 5, dtype=np.int64)))