Backport PR #28680: BUG: Fix RangeIndex.get_indexer for decreasing RangeIndex (#28745)

Author: meeseeksmachine

doc/source/whatsnew/v0.25.2.rst

@@ -50,6 +50,7 @@ Indexing
 ^^^^^^^^
 
 - Fix regression in :meth:`DataFrame.reindex` not following ``limit`` argument (:issue:`28631`).
+- Fix regression in :meth:`RangeIndex.get_indexer` for decreasing :class:`RangeIndex` where target values may be improperly identified as missing/present (:issue:`28678`)
 -
 -
 -

pandas/core/indexes/range.py

@@ -388,8 +388,9 @@ def get_indexer(self, target, method=None, limit=None, tolerance=None):
         if self.step > 0:
             start, stop, step = self.start, self.stop, self.step
         else:
-            # Work on reversed range for simplicity:
-            start, stop, step = (self.stop - self.step, self.start + 1, -self.step)
+            # GH 28678: work on reversed range for simplicity
+            reverse = self._range[::-1]
+            start, stop, step = reverse.start, reverse.stop, reverse.step
 
         target_array = np.asarray(target)
         if not (is_integer_dtype(target_array) and target_array.ndim == 1):

pandas/tests/indexes/test_range.py

@@ -424,6 +424,14 @@ def test_get_indexer_limit(self):
         expected = np.array([0, 1, 2, 3, 3, -1], dtype=np.intp)
         tm.assert_numpy_array_equal(result, expected)
 
+    @pytest.mark.parametrize("stop", [0, -1, -2])
+    def test_get_indexer_decreasing(self, stop):
+        # GH 28678
+        index = RangeIndex(7, stop, -3)
+        result = index.get_indexer(range(9))
+        expected = np.array([-1, 2, -1, -1, 1, -1, -1, 0, -1], dtype=np.intp)
+        tm.assert_numpy_array_equal(result, expected)
+
     def test_join_outer(self):
         # join with Int64Index
         other = Int64Index(np.arange(25, 14, -1))