Fix slicer assignment bug (#28131)

Author: RoeiRaz

doc/source/whatsnew/v1.0.0.rst

@@ -141,7 +141,7 @@ Interval
 Indexing
 ^^^^^^^^
 
--
+- Bug in assignment using a reverse slicer (:issue:`26939`)
 -
 
 Missing

pandas/core/indexers.py

@@ -226,6 +226,7 @@ def length_of_indexer(indexer, target=None) -> int:
         if step is None:
             step = 1
         elif step < 0:
+            start, stop = stop + 1, start + 1
             step = -step
         return (stop - start + step - 1) // step
     elif isinstance(indexer, (ABCSeries, ABCIndexClass, np.ndarray, list)):

pandas/tests/indexing/test_loc.py

@@ -1070,6 +1070,16 @@ def test_series_indexing_zerodim_np_array(self):
         result = s.loc[np.array(0)]
         assert result == 1
 
+    def test_loc_reverse_assignment(self):
+        # GH26939
+        data = [1, 2, 3, 4, 5, 6] + [None] * 4
+        expected = Series(data, index=range(2010, 2020))
+
+        result = pd.Series(index=range(2010, 2020))
+        result.loc[2015:2010:-1] = [6, 5, 4, 3, 2, 1]
+
+        tm.assert_series_equal(result, expected)
+
 
 def test_series_loc_getitem_label_list_missing_values():
     # gh-11428