feat(python): Raise if expression passed as scalar to DataFrame const…

…ructor (#12916)

py-polars/polars/utils/_construction.py

@@ -814,6 +814,14 @@ def _expand_dict_scalars(
     """Expand any scalar values in dict data (propagate literal as array)."""
     updated_data = {}
     if data:
+        if any(isinstance(val, pl.Expr) for val in data.values()):
+            raise TypeError(
+                "passing Expr objects to the DataFrame constructor is not supported"
+                "\n\nHint: Try evaluating the expression first using `select`,"
+                " or if you meant to create an Object column containing expressions,"
+                " pass a list of Expr objects instead."
+            )
+
         dtypes = schema_overrides or {}
         data = _expand_dict_data(data, dtypes)
         array_len = max((arrlen(val) or 0) for val in data.values())

py-polars/tests/unit/test_constructors.py

@@ -1466,3 +1466,17 @@ def test_list_null_constructor() -> None:
 def test_numpy_float_construction_av() -> None:
     np_dict = {"a": np.float64(1)}
     assert_frame_equal(pl.DataFrame(np_dict), pl.DataFrame({"a": 1.0}))
+
+
+def test_df_init_dict_raise_on_expression_input() -> None:
+    with pytest.raises(
+        TypeError,
+        match="passing Expr objects to the DataFrame constructor is not supported",
+    ):
+        pl.DataFrame({"a": pl.int_range(0, 3)})
+    with pytest.raises(TypeError):
+        pl.DataFrame({"a": pl.int_range(0, 3), "b": [3, 4, 5]})
+
+    # Passing a list of expressions is allowed
+    df = pl.DataFrame({"a": [pl.int_range(0, 3)]})
+    assert df.get_column("a").dtype == pl.Object