trying new patch location and logic & revamped test infra

Author: akkik04

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