REGR: Fixed slicing DatetimeIndex with date (#31521) (#31619)

Co-authored-by: Tom Augspurger <TomAugspurger@users.noreply.github.com>

Author: jorisvandenbossche

doc/source/whatsnew/v1.0.1.rst

@@ -77,6 +77,9 @@ Interval
 Indexing
 ^^^^^^^^
 
+
+- Fixed regression when indexing a ``Series`` or ``DataFrame`` indexed by ``DatetimeIndex`` with a slice containg a :class:`datetime.date` (:issue:`31501`)
+- Fixed regression in :class:`DataFrame` setting values with a slice (e.g. ``df[-4:] = 1``) indexing by label instead of position (:issue:`31469`)
 -
 -
 - Bug where assigning to a :class:`Series` using a IntegerArray / BooleanArray as a mask would raise ``TypeError`` (:issue:`31446`)

pandas/core/indexes/datetimes.py

@@ -1,4 +1,4 @@
-from datetime import datetime, time, timedelta, tzinfo
+from datetime import date, datetime, time, timedelta, tzinfo
 import operator
 from typing import Optional
 import warnings
@@ -804,6 +804,13 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
         if isinstance(start, time) or isinstance(end, time):
             raise KeyError("Cannot mix time and non-time slice keys")
 
+        # Pandas supports slicing with dates, treated as datetimes at midnight.
+        # https://github.com/pandas-dev/pandas/issues/31501
+        if isinstance(start, date) and not isinstance(start, datetime):
+            start = datetime.combine(start, time(0, 0))
+        if isinstance(end, date) and not isinstance(end, datetime):
+            end = datetime.combine(end, time(0, 0))
+
         try:
             return Index.slice_indexer(self, start, end, step, kind=kind)
         except KeyError:

pandas/tests/indexing/test_datetime.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import date, datetime, timedelta
 
 from dateutil import tz
 import numpy as np
@@ -350,3 +350,23 @@ def test_loc_label_slicing(self):
         expected = ser.iloc[:-1]
 
         tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "slice_, positions",
+        [
+            [slice(date(2018, 1, 1), None), [0, 1, 2]],
+            [slice(date(2019, 1, 2), None), [2]],
+            [slice(date(2020, 1, 1), None), []],
+            [slice(None, date(2020, 1, 1)), [0, 1, 2]],
+            [slice(None, date(2019, 1, 1)), [0]],
+        ],
+    )
+    def test_getitem_slice_date(self, slice_, positions):
+        # https://github.com/pandas-dev/pandas/issues/31501
+        s = pd.Series(
+            [0, 1, 2],
+            pd.DatetimeIndex(["2019-01-01", "2019-01-01T06:00:00", "2019-01-02"]),
+        )
+        result = s[slice_]
+        expected = s.take(positions)
+        tm.assert_series_equal(result, expected)