BUG: Fixed partial slicing on monotonic index with dupes

Fixed an edge case in partial string indexing where we would incorrectly flip
the endpoint on a slice, since we checked for monotonicity when we needed strict
monotonicity.

Closes #16515
xref dask/dask#2389

Author: TomAugspurger

doc/source/whatsnew/v0.20.2.txt

@@ -63,7 +63,7 @@ Indexing
 ^^^^^^^^
 
 - Bug in ``DataFrame.reset_index(level=)`` with single level index (:issue:`16263`)
-
+- Bug in partial string indexing with a monotonic, but not strictly-monotonic, index incorrectly reversing the slice bounds (:issue:`16515`)
 
 I/O
 ^^^

pandas/core/indexes/datetimes.py

@@ -1472,7 +1472,7 @@ def _maybe_cast_slice_bound(self, label, side, kind):
             # the bounds need swapped if index is reverse sorted and has a
             # length > 1 (is_monotonic_decreasing gives True for empty
             # and length 1 index)
-            if self.is_monotonic_decreasing and len(self) > 1:
+            if self.is_strictly_monotonic_decreasing and len(self) > 1:
                 return upper if side == 'left' else lower
             return lower if side == 'left' else upper
         else:

pandas/tests/indexes/datetimes/test_datetime.py

@@ -771,3 +771,10 @@ def test_slice_bounds_empty(self):
         left = empty_idx._maybe_cast_slice_bound('2015-01-02', 'left', 'loc')
         exp = Timestamp('2015-01-02 00:00:00')
         assert left == exp
+
+    def test_slice_duplicate_monotonic(self):
+        # https://github.com/pandas-dev/pandas/issues/16515
+        idx = pd.DatetimeIndex(['2017', '2017'])
+        result = idx._maybe_cast_slice_bound('2017-01-01', 'left', 'loc')
+        expected = Timestamp('2017-01-01')
+        assert result == expected