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

Author: mapehe

pandas/core/indexes/datetimelike.py

@@ -342,7 +342,7 @@ 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 type(res) == slice or np.any(res + 1)
         except (KeyError, TypeError, ValueError):
             return False
 

pandas/tests/indexing/test_datetime.py

@@ -252,3 +252,11 @@ def test_series_partial_set_period(self):
                                         check_stacklevel=False):
             result = ser.loc[keys]
         tm.assert_series_equal(result, exp)
+
+    def test_monotone_DTI_indexing_bug(self):
+        # GH 19362
+
+        df = pd.DataFrame({'A': [1, 2, 3]},
+                          index=pd.date_range('20170101',
+                                              periods=3)[::-1])
+        df.loc['2017-01-03']