Bug in setitem raising ValueError with row-slice indexer on df with list on rhs (#41268)

Author: phofl

doc/source/whatsnew/v1.3.0.rst

@@ -752,6 +752,7 @@ Indexing
 - Bug in :meth:`RangeIndex.append` where a single object of length 1 was concatenated incorrectly (:issue:`39401`)
 - Bug in setting ``numpy.timedelta64`` values into an object-dtype :class:`Series` using a boolean indexer (:issue:`39488`)
 - Bug in setting numeric values into a into a boolean-dtypes :class:`Series` using ``at`` or ``iat`` failing to cast to object-dtype (:issue:`39582`)
+- Bug in :meth:`DataFrame.__setitem__` and :meth:`DataFrame.iloc.__setitem__` raising ``ValueError`` when trying to index with a row-slice and setting a list as values (:issue:`40440`)
 - Bug in :meth:`DataFrame.loc.__setitem__` when setting-with-expansion incorrectly raising when the index in the expanding axis contains duplicates (:issue:`40096`)
 - Bug in :meth:`DataFrame.loc` incorrectly matching non-boolean index elements (:issue:`20432`)
 - Bug in :meth:`Series.__delitem__` with ``ExtensionDtype`` incorrectly casting to ``ndarray`` (:issue:`40386`)

pandas/core/indexers.py

@@ -180,7 +180,8 @@ def check_setitem_lengths(indexer, value, values) -> bool:
 
     elif isinstance(indexer, slice):
         if is_list_like(value):
-            if len(value) != length_of_indexer(indexer, values):
+            if len(value) != length_of_indexer(indexer, values) and values.ndim == 1:
+                # In case of two dimensional value is used row-wise and broadcasted
                 raise ValueError(
                     "cannot set using a slice indexer with a "
                     "different length than the value"

pandas/tests/frame/indexing/test_setitem.py

@@ -791,6 +791,34 @@ def test_setitem_slice_position(self):
         expected = DataFrame(arr)
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize("indexer", [tm.setitem, tm.iloc])
+    @pytest.mark.parametrize("box", [Series, np.array, list])
+    @pytest.mark.parametrize("n", [1, 2, 3])
+    def test_setitem_broadcasting_rhs(self, n, box, indexer):
+        # GH#40440
+        # TODO: Add pandas array as box after GH#40933 is fixed
+        df = DataFrame([[1, 3, 5]] + [[2, 4, 6]] * n, columns=["a", "b", "c"])
+        indexer(df)[1:] = box([10, 11, 12])
+        expected = DataFrame([[1, 3, 5]] + [[10, 11, 12]] * n, columns=["a", "b", "c"])
+        tm.assert_frame_equal(df, expected)
+
+    @pytest.mark.parametrize("indexer", [tm.setitem, tm.iloc])
+    @pytest.mark.parametrize("box", [Series, np.array, list])
+    @pytest.mark.parametrize("n", [1, 2, 3])
+    def test_setitem_broadcasting_rhs_mixed_dtypes(self, n, box, indexer):
+        # GH#40440
+        # TODO: Add pandas array as box after GH#40933 is fixed
+        df = DataFrame(
+            [[1, 3, 5], ["x", "y", "z"]] + [[2, 4, 6]] * n, columns=["a", "b", "c"]
+        )
+        indexer(df)[1:] = box([10, 11, 12])
+        expected = DataFrame(
+            [[1, 3, 5]] + [[10, 11, 12]] * (n + 1),
+            columns=["a", "b", "c"],
+            dtype="object",
+        )
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameSetItemCallable:
     def test_setitem_callable(self):