API: map() on Index returns an Index, not array

[ENH] Using _shallow_copy_with_infer, additional test for type change and attribute conservation

TST: Tests for various index types

REF: Move to tm.assert_index_equal

DOC: docstring for the map function

Author: jrings

doc/source/whatsnew/v0.18.1.txt

@@ -408,6 +408,8 @@ New behaviour:
 
    np.cumsum(sp, axis=0)
 
+- ``map`` on an ``Index`` now returns an ``Index``, not an array (:issue:`12766`)
+
 .. _whatsnew_0181.apply_resample:
 
 Using ``.apply`` on groupby resampling

pandas/indexes/base.py

@@ -2427,7 +2427,7 @@ def groupby(self, values):
 
     def map(self, mapper):
         """
-        Apply mapper function to its values.
+        Apply mapper function to an index
 
         Parameters
         ----------
@@ -2436,9 +2436,10 @@ def map(self, mapper):
 
         Returns
         -------
-        applied : array
+        An Index reflecting an appropriate Index with the mapper
+            function applied
         """
-        return self._arrmap(self.values, mapper)
+        return self._shallow_copy_with_infer(self._arrmap(self.values, mapper))
 
     def isin(self, values, level=None):
         """

pandas/tests/indexes/test_base.py

@@ -1744,6 +1744,41 @@ def test_string_index_repr(self):
 
                 self.assertEqual(coerce(idx), expected)
 
+    def test_map(self):
+
+        # Applying a function to the Index
+        df = pd.DataFrame([[0, 1], [2, 3]], columns=['c1', 'c2'], index=['i1', 'i2'])
+        df.index.name = "Numbering"
+        df.index = df.index.map(lambda x: x.upper())
+        self.assertTrue(df.index.equals(Index(['I1', 'I2'])))
+        self.assertEqual(df.index.name, "Numbering")
+
+        testIdx = self.unicodeIndex.map(lambda x: len(x))
+        tm.assert_index_equal(testIdx, Int64Index([10]*100))
+
+        testIdx = self.strIndex.map(lambda x: len(x))
+        tm.assert_index_equal(testIdx, Int64Index([10]*100))
+
+        testIdx = self.dateIndex.map(lambda x: x + timedelta(days=1))
+        tm.assert_index_equal(
+            testIdx, DatetimeIndex([dt + timedelta(days=1) for dt in tm.makeDateIndex(100)]))
+
+        testIdx = self.periodIndex.map(lambda x: x.to_timestamp())
+        tm.assert_index_equal(testIdx, self.dateIndex)
+
+        testIdx = self.intIndex.map(lambda x: str(x))
+        tm.assert_index_equal(testIdx, Index([str(i) for i in range(100)]))
+
+        testIdx = self.floatIndex.map(lambda x: -1 if x < 0 else 1)
+        self.assertEqual(len(testIdx), 100)
+        self.assertTrue(isinstance(testIdx, Int64Index))
+        self.assertTrue(set(testIdx == {-1, 1}))
+
+        testIdx = self.boolIndex.map(lambda x: not x)
+        tm.assert_index_equal(testIdx, Index([False, True]))
+
+        testIdx = self.catIndex.map(lambda x: len(x))
+
 
 class TestMixedIntIndex(Base, tm.TestCase):
     # Mostly the tests from common.py for which the results differ