fix(python): Simultaneous usage of named_expr and schema in `pl.s…

…truct` (#17768)

Co-authored-by: Casagrande, Sandro <[email protected]>

py-polars/polars/functions/as_datatype.py

@@ -598,7 +598,7 @@ def struct(
     pyexprs = parse_into_list_of_expressions(*exprs, **named_exprs)
 
     if schema:
-        if not exprs:
+        if not exprs and not named_exprs:
             # no columns or expressions provided; create one from schema keys
             expr = wrap_expr(
                 plr.as_struct(parse_into_list_of_expressions(list(schema.keys())))

py-polars/tests/unit/datatypes/test_struct.py

@@ -970,3 +970,11 @@ def test_struct_out_nullability_from_arrow() -> None:
 def test_empty_struct_raise() -> None:
     with pytest.raises(ValueError):
         pl.struct()
+
+
+def test_named_exprs() -> None:
+    df = pl.DataFrame({"a": 1})
+    schema = {"b": pl.Int64}
+    res = df.select(pl.struct(schema=schema, b=pl.col("a")))
+    assert res.to_dict(as_series=False) == {"b": [{"b": 1}]}
+    assert res.schema["b"] == pl.Struct(schema)