Merge pull request #9996 from slonik-az/bugfix/issue-9995

BUGFIX: length_of_indexer() can return incorrect values that break slice assignments

Author: shoyer

doc/source/whatsnew/v0.16.1.txt

@@ -205,6 +205,7 @@ Bug Fixes
 - Bug in ``to_msgpack`` and ``read_msgpack`` zlib and blosc compression support (:issue:`9783`)
 
 - Bug ``GroupBy.size`` doesn't attach index name properly if grouped by ``TimeGrouper`` (:issue:`9925`)
+- Bug causing an exception in slice assignments because ``length_of_indexer`` returns wrong results (:issue:`9995`)
 
 
 

pandas/core/indexing.py

@@ -1591,8 +1591,8 @@ def length_of_indexer(indexer, target=None):
         if step is None:
             step = 1
         elif step < 0:
-            step = abs(step)
-        return (stop - start) / step
+            step = -step
+        return (stop - start + step-1) // step
     elif isinstance(indexer, (ABCSeries, Index, np.ndarray, list)):
         return len(indexer)
     elif not is_list_like_indexer(indexer):

pandas/tests/test_indexing.py

@@ -1438,6 +1438,13 @@ def test_iloc_setitem_series(self):
         result = s.iloc[:4]
         assert_series_equal(result, expected)
 
+        s= Series([-1]*6)
+        s.iloc[0::2]= [0,2,4]
+        s.iloc[1::2]= [1,3,5]
+        result  = s
+        expected= Series([0,1,2,3,4,5])
+        assert_series_equal(result, expected)
+
     def test_iloc_setitem_list_of_lists(self):
 
         # GH 7551