code: def test_forecaster_index_step_stored():
      """
      Test serie without DatetimeIndex, step is stored in forecaster.index_freq.
      """
      y = pd.Series(data=np.arange(10))
      forecaster = ForecasterSarimax(regressor=Sarimax(order=(1, 0, 0)))
      forecaster.fit(y=y)
      expected = y.index.step
      results = forecaster.index_freq
  
      assert results == expected
  
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 3 for calling a global function with example usage: test_forecaster_index_step_stored() without return types