BUG: fix to_datetime(dti, utc=True) (#27733)

* BUG: fix to_datetime(dti, utc=True)

* whatsnew, suggested edits

* parametrize

* backquotes

Author: jbrockmendel

doc/source/whatsnew/v0.25.1.rst

@@ -31,7 +31,7 @@ Categorical
 
 Datetimelike
 ^^^^^^^^^^^^
-
+- Bug in :func:`to_datetime` where passing a timezone-naive :class:`DatetimeArray` or :class:`DatetimeIndex` and ``utc=True`` would incorrectly return a timezone-naive result (:issue:`27733`)
 -
 -
 -

pandas/core/tools/datetimes.py

@@ -334,6 +334,9 @@ def _convert_listlike_datetimes(
                 return DatetimeIndex(arg, tz=tz, name=name)
             except ValueError:
                 pass
+        elif tz:
+            # DatetimeArray, DatetimeIndex
+            return arg.tz_localize(tz)
 
         return arg
 

pandas/tests/indexes/datetimes/test_tools.py

@@ -1620,6 +1620,18 @@ def test_dayfirst(self, cache):
         tm.assert_index_equal(expected, idx5)
         tm.assert_index_equal(expected, idx6)
 
+    @pytest.mark.parametrize("klass", [DatetimeIndex, DatetimeArray])
+    def test_to_datetime_dta_tz(self, klass):
+        # GH#27733
+        dti = date_range("2015-04-05", periods=3).rename("foo")
+        expected = dti.tz_localize("UTC")
+
+        obj = klass(dti)
+        expected = klass(expected)
+
+        result = to_datetime(obj, utc=True)
+        tm.assert_equal(result, expected)
+
 
 class TestGuessDatetimeFormat:
     @td.skip_if_not_us_locale