trying new patch location and logic & revamped test infra

pandas/core/frame.py

@@ -5501,29 +5501,31 @@ def _sanitize_column(self, value) -> tuple[ArrayLike, BlockValuesRefs | None]:
  return _reindex_for_setitem(value, self.index)
 
  if is_list_like(value):
- # GH#61026: this method is only used for *single-column* assignment.
- # Reject 2D/3D arrays here, except the (n, 1) case which we treat as 1D.
- if isinstance(value, np.ndarray) and value.ndim > 1:
- if value.ndim == 2:
- if value.shape[1] == 1:
- # (n, 1) → length-n 1D array
- value = value[:, 0]
- else:
- # More than one column: users should use df[[...]] = value
- raise ValueError(
- "Setting a DataFrame column with a 2D array requires "
- f"shape (n, 1); got shape {value.shape}."
- )
- else:
- # ndim >= 3
- raise ValueError(
- f"Setting a DataFrame column with ndim {value.ndim} "
- "array is not supported."
- )
-
  com.require_length_match(value, self.index)
 
- return sanitize_array(value, self.index, copy=True, allow_2d=True), None
+ # GH#61026: special-case 2D inputs for single-column assignment.
+ # - accept shape (n, 1) by flattening to 1D
+ # - disallow 2D *object* arrays with more than one column, since those
+ # correspond to a single column key and should be rejected
+ arr = value
+
+ # np.matrix is always 2D; gonna convert to regular ndarray
+ if isinstance(arr, np.matrix):
+ arr = np.asarray(arr)
+
+ if isinstance(arr, np.ndarray) and arr.ndim == 2:
+ if arr.shape[1] == 1:
+ # treating (n, 1) as a length-n 1D array
+ arr = arr[:, 0]
+ elif arr.dtype == object:
+ # single-column setitem with a 2D object array is not allowed.
+ msg = (
+ "Setting a DataFrame column with a 2D array requires "
+ f"shape (n, 1); got shape {arr.shape}."
+ )
+ raise ValueError(msg)
+ subarr = sanitize_array(arr, self.index, copy=True, allow_2d=True)
+ return subarr, None
 
  @property
  def _series(self):

pandas/tests/frame/indexing/test_setitem.py

@@ -816,6 +816,24 @@ def test_setitem_index_object_dtype_not_inferring(self):
  )
  tm.assert_frame_equal(df, expected)
 
+ def test_setitem_2d_object_array(self):
+ # GH#61026
+ df = DataFrame(
+ {
+ "c1": [1, 2, 3, 4, 5],
+ }
+ )
+
+ arr = np.array([["A"], ["B"], ["C"], ["D"], ["E"]], dtype=object)
+ df["c1"] = arr
+
+ expected = DataFrame(
+ {
+ "c1": ["A", "B", "C", "D", "E"],
+ }
+ )
+ tm.assert_frame_equal(df, expected)
+
 
 class TestSetitemTZAwareValues:
  @pytest.fixture