Regression in loc setitem raising ValueError when setting array as cell value (#43450)

Author: phofl

doc/source/whatsnew/v1.3.3.rst

@@ -22,6 +22,7 @@ Fixed regressions
 - Fixed regression in :meth:`merge` where ``on`` columns with ``ExtensionDtype`` or ``bool`` data types were cast to ``object`` in ``right`` and ``outer`` merge (:issue:`40073`)
 - Fixed regression in :meth:`RangeIndex.where` and :meth:`RangeIndex.putmask` raising ``AssertionError`` when result did not represent a :class:`RangeIndex` (:issue:`43240`)
 - Fixed regression in :meth:`read_parquet` where the ``fastparquet`` engine would not work properly with fastparquet 0.7.0 (:issue:`43075`)
+- Fixed regression in :meth:`DataFrame.loc.__setitem__` raising ``ValueError`` when setting array as cell value (:issue:`43422`)
 - Fixed regression in :func:`is_list_like` where objects with ``__iter__`` set to ``None`` would be identified as iterable (:issue:`43373`)
 - Fixed regression in :meth:`.Resampler.aggregate` when used after column selection would raise if ``func`` is a list of aggregation functions (:issue:`42905`)
 - Fixed regression in :meth:`DataFrame.corr` where Kendall correlation would produce incorrect results for columns with repeated values (:issue:`43401`)

pandas/core/dtypes/cast.py

@@ -2185,6 +2185,11 @@ def can_hold_element(arr: ArrayLike, element: Any) -> bool:
         # ExtensionBlock._can_hold_element
         return True
 
+    # error: Non-overlapping equality check (left operand type: "dtype[Any]", right
+    # operand type: "Type[object]")
+    if dtype == object:  # type: ignore[comparison-overlap]
+        return True
+
     tipo = maybe_infer_dtype_type(element)
 
     if dtype.kind in ["i", "u"]:
@@ -2232,11 +2237,6 @@ def can_hold_element(arr: ArrayLike, element: Any) -> bool:
             return tipo.kind == "b"
         return lib.is_bool(element)
 
-    # error: Non-overlapping equality check (left operand type: "dtype[Any]", right
-    # operand type: "Type[object]")
-    elif dtype == object:  # type: ignore[comparison-overlap]
-        return True
-
     elif dtype.kind == "S":
         # TODO: test tests.frame.methods.test_replace tests get here,
         #  need more targeted tests.  xref phofl has a PR about this

pandas/tests/frame/indexing/test_indexing.py

@@ -1206,6 +1206,13 @@ def test_getitem_interval_index_partial_indexing(self):
         res = df.loc[:, 0.5]
         tm.assert_series_equal(res, expected)
 
+    def test_setitem_array_as_cell_value(self):
+        # GH#43422
+        df = DataFrame(columns=["a", "b"], dtype=object)
+        df.loc[0] = {"a": np.zeros((2,)), "b": np.zeros((2, 2))}
+        expected = DataFrame({"a": [np.zeros((2,))], "b": [np.zeros((2, 2))]})
+        tm.assert_frame_equal(df, expected)
+
 
 class TestDataFrameIndexingUInt64:
     def test_setitem(self, uint64_frame):