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

doc/source/whatsnew/v1.2.0.rst

@@ -677,6 +677,8 @@ 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 a listlike ``np.array``, ``list`` or ``tuple`` in an ``object`` Series of equal length (:issue:`37748`)
+- Bug in :meth:`Series.loc` and :meth:`Series.iloc` setting all the values of an ``object`` Series with those of a listlike ``ExtensionArray`` instead of inserting it (:issue:`38271`)
 
 Missing
 ^^^^^^^

pandas/_testing.py

@@ -1755,6 +1755,18 @@ def assert_equal(left, right, **kwargs):
         raise NotImplementedError(type(left))
 
 
+def assert_python_equal(left, right):
+    """
+    Check left and right are equal w.r.t the ``==`` operator.
+
+    Parameters
+    ----------
+    left : object
+    right : object
+    """
+    assert left == right
+
+
 def box_expected(expected, box_cls, transpose=True):
     """
     Helper function to wrap the expected output of a test in a given box_class.

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_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 +2072,32 @@ def test_loc_setitem_dt64tz_values(self):
         s2["a"] = expected
         result = s2["a"]
         assert result == expected
+
+    @pytest.mark.parametrize(
+        "array_fn,assert_fn",
+        [
+            (np.zeros, tm.assert_numpy_array_equal),
+            (pd.array, tm.assert_extension_array_equal),
+            (list, tm.assert_python_equal),
+            (tuple, tm.assert_python_equal),
+        ],
+    )
+    @pytest.mark.parametrize("size", [0, 4, 5, 6])
+    def test_loc_iloc_setitem_with_listlike(self, size, array_fn, assert_fn):
+        # GH37748
+        # testing insertion, in Series of size N (here 5), of listlike
+        # of size  0, N-1, N, N+1
+
+        expected = array_fn([0] * size)
+
+        ser = Series(0, index=list("abcde"), dtype="object")
+
+        ser.loc["a"] = expected
+        result = ser[0]
+        assert_fn(result, expected)
+
+        ser = Series(0, index=list("abcde"), dtype="object")
+
+        ser.iloc[0] = expected
+        result = ser[0]
+        assert_fn(result, expected)