code: def forecast_index(request):
      idx = pd.date_range("2000-01-01", periods=400, freq="B")
      if request.param is None:
          return None
      elif request.param == "list":
          return list(idx)
      return idx
  
imports: from statsmodels.compat.pandas import MONTH_END, PD_LT_1_0_0, QUARTER_END, YEAR_END, is_int_index
  from statsmodels.compat.pytest import pytest_warns
  from collections.abc import Hashable
  import numpy
  import pandas
  import pytest
  from statsmodels.tsa.deterministic import CalendarFourier, CalendarSeasonality, CalendarTimeTrend, DeterministicProcess, DeterministicTerm, Fourier, Seasonality, TimeTrend
label_desc: Use image node_id 3 for calling a global function with example usage: forecast_index(request) and returns: idx, None, list