code: def test_cumulative_returns_less_costs(self):
      returns = pd.Series(
          [0.1] * 3, index=pd.date_range("2017-01-01", periods=3)
      )
      cost = pd.Series([0.001] * len(returns), index=returns.index)
  
      expected_returns = pd.Series(
          [0.1, 0.21, 0.331], index=returns.index
      )
      pd.util.testing.assert_series_equal(
          expected_returns,
          _cumulative_returns_less_costs(returns, None),
      )
  
      expected_returns = pd.Series(
          [0.099000, 0.207801, 0.327373], index=returns.index
      )
      pd.util.testing.assert_series_equal(
          expected_returns,
          _cumulative_returns_less_costs(returns, cost),
      )
  
file_path: pyfolio/pyfolio/tests/test_perf_attrib.py
imports: import numpy
  import pandas
  import unittest
  import warnings
  import empyrical
  from pyfolio.perf_attrib import perf_attrib, create_perf_attrib_stats, _cumulative_returns_less_costs
label_desc: Use image node_id 5 for calling the PerfAttribTestCase obj's underlying member method code with example usage: obj.test_cumulative_returns_less_costs() without return types