code: def test_fit_extended_index_stored(regressor, y, idx):
      """
      Test that values of self.regressor.arima_res_.fittedvalues.index are
      stored after fitting in forecaster.extended_index.
      """
      forecaster = ForecasterSarimax(regressor=regressor)
      forecaster.fit(y=y)
  
      pd.testing.assert_index_equal(forecaster.extended_index, idx)
  
file_path: skforecast/skforecast/ForecasterSarimax/tests/test_fit.py
imports: import re
  import pytest
  import numpy
  import pandas
  from pmdarima.arima import ARIMA
  from skforecast.Sarimax import Sarimax
  from skforecast.ForecasterSarimax import ForecasterSarimax
  from .fixtures_ForecasterSarimax import y
  from .fixtures_ForecasterSarimax import y_datetime
label_desc: Use image node_id 5 for calling a global function with example usage: test_fit_extended_index_stored(regressor, y, idx) without return types