code: def test_expanding_sum() -> None:
      df = pl.DataFrame(
          {
              "x": [0, 1, 2],
              "y_1": [1.1, 2.2, 3.3],
              "y_2": [1.0, 2.5, 3.5],
          }
      )
  
      result = df.with_columns(
          pl.sum_horizontal(pl.col(r"^y_.*$")).alias("y_sum")
      )["y_sum"]
      assert result.to_list() == [2.1, 4.7, 6.8]
  
imports: from __future__ import annotations
  import datetime
  from typing import Any
  import pytest
  import polars
  from polars.testing import assert_frame_equal, assert_series_equal
label_desc: Use image node_id 13 for calling a global function with example usage: test_expanding_sum() without return types