BUG: Inserting array of same size with Series.loc raises ValueError

doc/source/whatsnew/v1.2.0.rst

@@ -677,6 +677,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` and :meth:`DataFrame.__getitem__`  raising ``KeyError`` when columns were :class:`MultiIndex` with only one level (:issue:`29749`)
 - Bug in :meth:`Series.__getitem__` and :meth:`DataFrame.__getitem__` raising blank ``KeyError`` without missing keys for :class:`IntervalIndex` (:issue:`27365`)
 - Bug in setting a new label on a :class:`DataFrame` or :class:`Series` with a :class:`CategoricalIndex` incorrectly raising ``TypeError`` when the new label is not among the index's categories (:issue:`38098`)
+- Bug in :meth:`Series.loc` and :meth:`Series.iloc` raising ``ValueError`` when inserting an array in a ``object`` Series of equal length (:issue:`37748`)
 
 Missing
 ^^^^^^^

pandas/core/indexers.py

@@ -79,6 +79,9 @@ def is_scalar_indexer(indexer, ndim: int) -> bool:
     -------
     bool
     """
+    if ndim == 1 and is_integer(indexer):
+        # GH37748: allow indexer to be an integer for Series
+        return True
     if isinstance(indexer, tuple):
         if len(indexer) == ndim:
             return all(

pandas/tests/indexing/test_iloc.py

@@ -1042,3 +1042,14 @@ def test_iloc_setitem_pure_position_based(self):
         ser1.iloc[1:3] = ser2.iloc[1:3]
         expected = Series([1, 5, 6])
         tm.assert_series_equal(ser1, expected)
+
+    @pytest.mark.parametrize("size", [0, 4, 5, 6])
+    def test_iloc_setitem_with_array(self, size):
+        # GH37748
+        ser = Series(0, index=range(5), dtype="object")
+
+        expected = np.zeros(size)
+        ser.iloc[0] = expected
+        result = ser[0]
+
+        tm.assert_numpy_array_equal(result, expected)

pandas/tests/indexing/test_indexers.py

@@ -28,6 +28,12 @@ def test_is_scalar_indexer():
 
     assert not is_scalar_indexer(slice(None), 1)
 
+    indexer = 0
+    assert is_scalar_indexer(indexer, 1)
+
+    indexer = (0,)
+    assert is_scalar_indexer(indexer, 1)
+
 
 class TestValidateIndices:
     def test_validate_indices_ok(self):

pandas/tests/indexing/test_loc.py

@@ -2072,3 +2072,14 @@ def test_loc_setitem_dt64tz_values(self):
         s2["a"] = expected
         result = s2["a"]
         assert result == expected
+
+    @pytest.mark.parametrize("size", [0, 4, 5, 6])
+    def test_loc_setitem_with_array(self, size):
+        # GH37748
+        ser = Series(0, index=list("abcde"), dtype="object")
+
+        expected = np.zeros(size)
+        ser.loc["a"] = expected
+        result = ser[0]
+
+        tm.assert_numpy_array_equal(result, expected)