code: def test_cum_should_not_side_effect():
      _timeseries_df = timeseries_df.copy()
      pp.cum(
          df=timeseries_df,
          columns={"y": "y2"},
          operator="sum",
      )
      assert _timeseries_df.equals(timeseries_df)
  
file_path: superset/tests/unit_tests/pandas_postprocessing/test_cum.py
imports: import pandas
  import pytest
  from superset.exceptions import InvalidPostProcessingError
  from superset.utils import pandas_postprocessing
  from superset.utils.pandas_postprocessing.utils import FLAT_COLUMN_SEPARATOR
  from tests.unit_tests.fixtures.dataframes import multiple_metrics_df, single_metric_df, timeseries_df, timeseries_with_gap_df
  from tests.unit_tests.pandas_postprocessing.utils import series_to_list
label_desc: Use image node_id 1 for calling a global function with example usage: test_cum_should_not_side_effect() without return types