proposed fix for issue #61026

Author: akkik04

pandas/core/construction.py

@@ -611,6 +611,24 @@ def sanitize_array(
             data = data.A
 
         if dtype is None:
+            # GH#61026: special-case 2D+ object ndarrays when dtype is None.
+            if data.dtype == object and data.ndim > 1:
+                if data.ndim == 2 and data.shape[1] == 1:
+                    # allow assigning a (n, 1) object array to a single column, flatten it:
+                    data = data[:, 0]
+                elif data.ndim == 2:
+                    # More than one column but caller is behaving as if this is a single-column assignment.
+                    raise ValueError(
+                        "Setting a DataFrame column with a 2D object array "
+                        f"requires shape (n, 1); got shape {data.shape}."
+                    )
+                else:
+                    # ndim >= 3
+                    raise ValueError(
+                        f"Setting a DataFrame column with ndim {data.ndim} "
+                        "object array is not supported."
+                    )
+            
             subarr = data
             if data.dtype == object and infer_object:
                 subarr = lib.maybe_convert_objects(

pandas/tests/frame/indexing/test_setitem_2d_object.py

@@ -0,0 +1,52 @@
+import numpy as np
+import pandas as pd
+import pandas._testing as tm
+import pytest
+
+
+class TestSetItem2DObjectArray:
+    # GH#61026
+    def test_setitem_2d_object_array_single_column_unravel(self):
+        df = pd.DataFrame({"c1": [1, 2, 3]})
+        t = np.array([["A"], ["B"], ["C"]], dtype=object)
+
+        df["c1"] = t
+
+        expected = pd.Series(["A", "B", "C"], name="c1")
+        tm.assert_series_equal(df["c1"], expected)
+
+    # GH#61026
+    def test_setitem_2d_object_array_wrong_shape_raises(self):
+        df = pd.DataFrame({"c1": [1, 2, 3]})
+        t = np.array([["A", "B"], ["C", "D"], ["E", "F"]], dtype=object)
+
+        with pytest.raises(ValueError, match="requires shape"):
+            df["c1"] = t
+
+    # GH#61026
+    def test_setitem_3d_object_array_raises(self):
+        df = pd.DataFrame({"c1": [1, 2, 3]})
+        t = np.array([[["A"]], [["B"]], [["C"]]], dtype=object)
+
+        with pytest.raises(ValueError, match="ndim 3"):
+            df["c1"] = t
+
+    # GH#61026
+    def test_setitem_2d_string_array_regression(self):
+        df = pd.DataFrame({"c1": [1, 2, 3]})
+        t = np.array([["A"], ["B"], ["C"]])  # dtype '<U1'
+
+        df["c1"] = t
+
+        # just checking values; internal mask shape isn't part of the contract.
+        assert list(df["c1"]) == ["A", "B", "C"]
+
+    # GH#61026
+    def test_setitem_multicolumn_object_array_still_ok(self):
+        df = pd.DataFrame({"c1": [1, 2, 3], "c2": [4, 5, 6]})
+        t = np.array([["A", "X"], ["B", "Y"], ["C", "Z"]], dtype=object)
+
+        df[["c1", "c2"]] = t
+
+        expected = pd.DataFrame({"c1": ["A", "B", "C"], "c2": ["X", "Y", "Z"]})
+        tm.assert_frame_equal(df, expected)