Modify whatsnew and make new wrapper function to handle UTC conversion

Author: mroeschke

doc/source/whatsnew/v0.21.0.txt

@@ -208,10 +208,10 @@ The configuration option ``pd.options.mode.use_inf_as_null`` is deprecated, and
 UTC Localization with Series
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Previously, :func:`to_datetime` did not localize datetime ``Series`` data as when ``utc=True`` was passed. Now, :func:`to_datetime`
-will correctly localize `Series` with a `datetime64[ns, UTC]` data type. (:issue:`6415`)
+:func:`to_datetime` now correctly localizes datetime ``Series`` data as when ``utc=True`` was passed with 
+a `datetime64[ns, UTC]` data type. (:issue:`6415`)
 
-  Old Behavior
+  Previous Behavior
 
   .. ipython:: python
 

pandas/core/tools/datetimes.py

@@ -340,6 +340,14 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
 
     tz = 'utc' if utc else None
 
+    def _maybe_convert_to_utc(arg, utc):
+        if utc:
+            if isinstance(arg, ABCSeries):
+                arg = arg.dt.tz_localize('UTC')
+            elif isinstance(arg, DatetimeIndex):
+                arg = arg.tz_convert(None).tz_localize('UTC')
+        return arg
+
     def _convert_listlike(arg, box, format, name=None, tz=tz):
 
         if isinstance(arg, (list, tuple)):
@@ -359,9 +367,7 @@ def _convert_listlike(arg, box, format, name=None, tz=tz):
                     return DatetimeIndex(arg, tz=tz, name=name)
                 except ValueError:
                     pass
-            from pandas import Series
-            if isinstance(arg, Series) and utc:
-                arg = arg.dt.tz_localize('utc')
+            arg = _maybe_convert_to_utc(arg, utc)
             return arg
 
         elif unit is not None:
@@ -438,12 +444,11 @@ def _convert_listlike(arg, box, format, name=None, tz=tz):
                     yearfirst=yearfirst,
                     require_iso8601=require_iso8601
                 )
-            from pandas import Series
             if is_datetime64_dtype(result) and box:
                 result = DatetimeIndex(result, tz=tz, name=name)
             # GH 6415
-            elif isinstance(arg, Series) and utc:
-                result = Series(result, name=name).dt.tz_localize('utc')
+            elif isinstance(arg, ABCSeries):
+                result = _maybe_convert_to_utc(Series(result, name=name), utc)
             return result
 
         except ValueError as e:

pandas/tests/io/test_sql.py

@@ -1321,18 +1321,19 @@ def check(col):
     def test_date_parsing(self):
         # No Parsing
         df = sql.read_sql_table("types_test_data", self.conn)
-
+        # Now that GH 6415 is fixed, dates are automatically parsed to UTC
+        utc_dtype = pd.core.dtypes.dtypes.DatetimeTZDtypeType
         df = sql.read_sql_table("types_test_data", self.conn,
                                 parse_dates=['DateCol'])
-        assert issubclass(df.DateCol.dtype.type, np.datetime64)
+        assert issubclass(df.DateCol.dtype.type, utc_dtype)
 
         df = sql.read_sql_table("types_test_data", self.conn,
                                 parse_dates={'DateCol': '%Y-%m-%d %H:%M:%S'})
-        assert issubclass(df.DateCol.dtype.type, np.datetime64)
+        assert issubclass(df.DateCol.dtype.type, utc_dtype)
 
         df = sql.read_sql_table("types_test_data", self.conn, parse_dates={
             'DateCol': {'format': '%Y-%m-%d %H:%M:%S'}})
-        assert issubclass(df.DateCol.dtype.type, np.datetime64)
+        assert issubclass(df.DateCol.dtype.type, utc_dtype)
 
         df = sql.read_sql_table(
             "types_test_data", self.conn, parse_dates=['IntDateCol'])