new fix

pandas/core/construction.py

@@ -566,14 +566,6 @@ def sanitize_array(
  # extract ndarray or ExtensionArray, ensure we have no NumpyExtensionArray
  data = extract_array(data, extract_numpy=True, extract_range=True)
 
- # GH#61026: when 2D input is allowed (e.g. DataFrame column assignment),
- # treat a (n, 1) numpy array as a 1D array of length n so downstream code
- # (including pyarrow-backed StringArray) always sees 1D.
- if allow_2d and isinstance(data, np.ndarray) and data.ndim == 2:
- rows, cols = data.shape
- if cols == 1:
- data = data[:, 0]
-
  if isinstance(data, np.ndarray) and data.ndim == 0:
  if dtype is None:
  dtype = data.dtype
@@ -619,24 +611,6 @@ def sanitize_array(
  data = data.A
 
  if dtype is None:
- # GH#61026: special-case 2D+ object ndarrays when dtype is None.
- if allow_2d and 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.
- data = data[:, 0]
- elif data.ndim == 2:
- # more than 1 column, not allowed.
- 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/core/frame.py

@@ -5501,7 +5501,28 @@ 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
 
  @property