BUG: fixes indexing with monotonic decreasing DTI (#19362)

doc/source/whatsnew/v0.23.0.txt

@@ -1073,6 +1073,7 @@ Indexing
 - Bug in :meth:`Index.difference` when taking difference of an ``Index`` with itself (:issue:`20040`)
 - Bug in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` in presence of entire rows of NaNs in the middle of values (:issue:`20499`).
 - Bug in :class:`IntervalIndex` where some indexing operations were not supported for overlapping or non-monotonic ``uint64`` data (:issue:`20636`)
+- Bug in indexing with monotonic decreasing datetimelike (:issue:`19362`)
 
 MultiIndex
 ^^^^^^^^^^

pandas/core/indexes/datetimelike.py

@@ -342,7 +342,8 @@ def _format_with_header(self, header, **kwargs):
     def __contains__(self, key):
         try:
             res = self.get_loc(key)
-            return is_scalar(res) or type(res) == slice or np.any(res)
+            return is_scalar(res) or isinstance(res, slice) or \
+                (is_list_like(res) and len(res))
         except (KeyError, TypeError, ValueError):
             return False
 

pandas/tests/indexes/datetimes/test_datetime.py

@@ -368,3 +368,14 @@ def test_factorize_dst(self):
     def test_unique(self, arr, expected):
         result = arr.unique()
         tm.assert_index_equal(result, expected)
+
+    def test_monotone_DTI_indexing_bug(self):
+        # GH 19362
+
+        df = pd.DataFrame({'A': [1, 2, 3]},
+                          index=pd.date_range('20170101',
+                                              periods=3)[::-1])
+        expected = pd.DataFrame({'A': 1},
+                                index=pd.date_range('20170103',
+                                                    periods=1))
+        tm.assert_frame_equal(df.loc['2017-01-03'], expected)