code: def test_duration_cum_sum() -> None:
      df = pl.DataFrame({"A": [timedelta(days=1), timedelta(days=2)]})
  
      assert df.select(pl.col("A").cum_sum()).to_dict(
          as_series=False
      ) == {"A": [timedelta(days=1), timedelta(days=3)]}
      assert df.schema["A"].is_(pl.Duration(time_unit="us"))
      for duration_dtype in (
          pl.Duration,
          pl.Duration(time_unit="ms"),
          pl.Duration(time_unit="ns"),
      ):
          assert df.schema["A"].is_(duration_dtype) is False
  
imports: from datetime import timedelta
  import polars
  from polars.testing import assert_frame_equal
label_desc: Use image node_id 1 for calling a global function with example usage: test_duration_cum_sum() without return types