code: def test_fold_regex_expand() -> None:
      df = pl.DataFrame(
          {
              "x": [0, 1, 2],
              "y_1": [1.1, 2.2, 3.3],
              "y_2": [1.0, 2.5, 3.5],
          }
      )
      assert df.with_columns(
          pl.fold(
              acc=pl.lit(0),
              function=lambda acc, x: acc + x,
              exprs=pl.col("^y_.*$"),
          ).alias("y_sum"),
      ).to_dict(as_series=False) == {
          "x": [0, 1, 2],
          "y_1": [1.1, 2.2, 3.3],
          "y_2": [1.0, 2.5, 3.5],
          "y_sum": [2.1, 4.7, 6.8],
      }
  
imports: import polars
  from polars.testing import assert_frame_equal
label_desc: Use image node_id 2 for calling a global function with example usage: test_fold_regex_expand() without return types