make tz_localize operate on values rather than categories

Author: Marco Gorelli

pandas/core/indexes/accessors.py

@@ -324,7 +324,9 @@ def __new__(cls, data):
 
         orig = data if is_categorical_dtype(data) else None
         if orig is not None:
-            data = Series(orig.values.categories, name=orig.name, copy=False)
+            data = Series(
+                orig.values.astype("datetime64[ns]"), name=orig.name, copy=False
+            )
 
         try:
             if is_datetime64_dtype(data.dtype):

pandas/tests/series/test_timezones.py

@@ -373,6 +373,17 @@ def test_tz_localize_convert_copy_inplace_mutate(self, copy, method, tz):
         )
         tm.assert_series_equal(result, expected)
 
+    def test_tz_localize_categorical(self, tz_aware_fixture):
+        # GH 27952
+        tz = tz_aware_fixture
+        datetimes = Series(
+            ["2019-01-01", "2019-01-01", "2019-01-02"], dtype="datetime64[ns]"
+        )
+        categorical = datetimes.astype("category")
+        result = categorical.dt.tz_localize(tz)
+        expected = datetimes.dt.tz_localize(tz)
+        tm.assert_series_equal(result, expected)
+
     def test_constructor_data_aware_dtype_naive(self, tz_aware_fixture):
         # GH 25843
         tz = tz_aware_fixture