fix(python): Fix edge case in DataFrame constructor data orientation …

…inference (#16975)

py-polars/polars/_utils/construction/dataframe.py

@@ -227,6 +227,10 @@ def _parse_schema_overrides(
                 col = col[0]
             column_names.append(col)
 
+    if n_expected is not None and len(column_names) != n_expected:
+        msg = "data does not match the number of columns"
+        raise ShapeError(msg)
+
     # determine column dtypes from schema and lookup_names
     lookup: dict[str, str] | None = (
         {
@@ -533,18 +537,11 @@ def _sequence_of_sequence_to_pydf(
     infer_schema_length: int | None,
 ) -> PyDataFrame:
     if orient is None:
-        # note: limit type-checking to smaller data; larger values are much more
-        # likely to indicate col orientation anyway, so minimise extra checks.
-        if len(first_element) > 1000:
-            orient = "col" if schema and len(schema) == len(data) else "row"
-        elif (schema is not None and len(schema) == len(data)) or not schema:
-            # check if element types in the first 'row' resolve to a single dtype.
-            row_types = {type(value) for value in first_element if value is not None}
-            if int in row_types and float in row_types:
-                row_types.discard(int)
-            orient = "col" if len(row_types) == 1 else "row"
-        else:
-            orient = "row"
+        orient = _infer_data_orientation(
+            first_element,
+            len_data=len(data),
+            len_schema=len(schema) if schema is not None else None,
+        )
 
     if orient == "row":
         column_names, schema_overrides = _unpack_schema(
@@ -555,13 +552,6 @@ def _sequence_of_sequence_to_pydf(
             if schema_overrides
             else {}
         )
-        if (
-            column_names
-            and len(first_element) > 0
-            and len(first_element) != len(column_names)
-        ):
-            msg = "the row data does not match the number of columns"
-            raise ShapeError(msg)
 
         unpack_nested = False
         for col, tp in local_schema_override.items():
@@ -589,7 +579,7 @@ def _sequence_of_sequence_to_pydf(
             )
         return pydf
 
-    if orient == "col" or orient is None:
+    elif orient == "col":
         column_names, schema_overrides = _unpack_schema(
             schema, schema_overrides=schema_overrides, n_expected=len(data)
         )
@@ -604,8 +594,34 @@ def _sequence_of_sequence_to_pydf(
         ]
         return PyDataFrame(data_series)
 
-    msg = f"`orient` must be one of {{'col', 'row', None}}, got {orient!r}"
-    raise ValueError(msg)
+    else:
+        msg = f"`orient` must be one of {{'col', 'row', None}}, got {orient!r}"
+        raise ValueError(msg)
+
+
+def _infer_data_orientation(
+    first_element: Sequence[Any] | np.ndarray[Any, Any],
+    len_data: int,
+    len_schema: int | None = None,
+) -> Orientation:
+    # Check if element types in the first 'row' resolve to a single dtype.
+    # Note: limit type-checking to smaller data; larger values are much more
+    # likely to indicate col orientation anyway, so minimize extra checks.
+    if len(first_element) <= 1000 and (len_schema is None or len_schema == len_data):
+        row_types = {type(value) for value in first_element if value is not None}
+        if int in row_types and float in row_types:
+            row_types.discard(int)
+        return "row" if len(row_types) > 1 else "col"
+
+    elif (
+        len_schema is not None
+        and len_schema == len(first_element)
+        and len_schema != len_data
+    ):
+        return "row"
+
+    else:
+        return "col"
 
 
 def _sequence_of_series_to_pydf(

py-polars/tests/unit/constructors/test_constructors.py

@@ -13,6 +13,7 @@
 from pydantic import BaseModel, Field, TypeAdapter
 
 import polars as pl
+from polars._utils.construction.dataframe import _infer_data_orientation
 from polars._utils.construction.utils import try_get_type_hints
 from polars.datatypes import PolarsDataType, numpy_char_code_to_dtype
 from polars.dependencies import dataclasses, pydantic
@@ -1622,3 +1623,13 @@ def test_array_construction() -> None:
     df = pl.from_dicts(rows, schema=schema)
     assert df.schema == schema
     assert df.rows() == [("a", [1, 2, 3]), ("b", [2, 3, 4])]
+
+
+def test_infer_data_orientation() -> None:
+    assert _infer_data_orientation([1], 1) == "col"
+    assert _infer_data_orientation([1, 2], 2) == "col"
+    assert _infer_data_orientation([1, 2], 2, 2) == "col"
+    assert _infer_data_orientation([1, 2, 3], 2) == "col"
+    assert _infer_data_orientation([1, 2, 3], 2, 2) == "col"
+    assert _infer_data_orientation([1, 2, 3], 2, 3) == "row"
+    assert _infer_data_orientation([1, "x"], 2) == "row"