FIX: Index._searchsorted_monotonic for decreasing indexes

Fixed an issue where Index._searchsorted_monotonic(..., side='right')
returns the left side position for monotonic decreasing indexes.  Issue had a downstream effect on scalar lookups in IntervalIndex as well.

Author: jschendel

doc/source/whatsnew/v0.21.0.txt

@@ -418,6 +418,7 @@ Indexing
 - Bug in ``.isin()`` in which checking membership in empty ``Series`` objects raised an error (:issue:`16991`)
 - Bug in ``CategoricalIndex`` reindexing in which specified indices containing duplicates were not being respected (:issue:`17323`)
 - Bug in intersection of ``RangeIndex`` with negative step (:issue:`17296`)
+- Bug in ``IntervalIndex`` where performing a scalar lookup fails for included right endpoints of non-overlapping monotonic decreasing indexes (:issue:`16417`, :issue:`17271`)
 
 I/O
 ^^^

pandas/core/indexes/base.py

@@ -3465,7 +3465,7 @@ def _searchsorted_monotonic(self, label, side='left'):
             # everything for it to work (element ordering, search side and
             # resulting value).
             pos = self[::-1].searchsorted(label, side='right' if side == 'left'
-                                          else 'right')
+                                          else 'left')
             return len(self) - pos
 
         raise ValueError('index must be monotonic increasing or decreasing')

pandas/tests/indexes/test_base.py

@@ -2119,6 +2119,27 @@ def test_intersect_str_dates(self):
 
         assert len(res) == 0
 
+    def test_searchsorted_monotonic(self):
+        # GH17271
+        idx_inc = Index([0, 2, 4])
+        idx_dec = Index([4, 2, 0])
+
+        # test included value.
+        assert idx_inc._searchsorted_monotonic(0, side='left') == 0
+        assert idx_inc._searchsorted_monotonic(0, side='right') == 1
+        assert idx_dec._searchsorted_monotonic(0, side='left') == 2
+        assert idx_dec._searchsorted_monotonic(0, side='right') == 3
+
+        # test non-included value.
+        for side in ('left', 'right'):
+            assert idx_inc._searchsorted_monotonic(1, side=side) == 1
+            assert idx_dec._searchsorted_monotonic(1, side=side) == 2
+
+        # non-monotonic should raise.
+        idx_bad = Index([0, 4, 2])
+        with pytest.raises(ValueError):
+            idx_bad._searchsorted_monotonic(3)
+
 
 class TestIndexUtils(object):
 

pandas/tests/indexing/test_interval.py

@@ -11,6 +11,9 @@ class TestIntervalIndex(object):
     def setup_method(self, method):
         self.s = Series(np.arange(5), IntervalIndex.from_breaks(np.arange(6)))
 
+        idx_dec = IntervalIndex.from_tuples([(2, 3), (1, 2), (0, 1)])
+        self.s_dec = Series(list('abc'), idx_dec)
+
     def test_loc_with_scalar(self):
 
         s = self.s
@@ -39,6 +42,9 @@ def test_loc_with_scalar(self):
         expected = s.iloc[2:5]
         tm.assert_series_equal(expected, s.loc[s >= 2])
 
+        # test endpoint of non-overlapping monotonic decreasing (GH16417)
+        assert self.s_dec.loc[3] == 'a'
+
     def test_getitem_with_scalar(self):
 
         s = self.s
@@ -67,6 +73,9 @@ def test_getitem_with_scalar(self):
         expected = s.iloc[2:5]
         tm.assert_series_equal(expected, s[s >= 2])
 
+        # test endpoint of non-overlapping monotonic decreasing (GH16417)
+        assert self.s_dec[3] == 'a'
+
     def test_with_interval(self):
 
         s = self.s