code: def test_output_preprocess_y_when_y_index_is_DatetimeIndex_and_has_frequency():
      """
      Test values returned by when y is a pandas Series DatetimeIndex and freq is
      not None.
      """
      y = pd.Series(
          data=np.arange(3),
          index=pd.date_range("1990-01-01", periods=3, freq="D"),
      )
      results = preprocess_y(y)
      expected = (
          np.arange(3),
          pd.DatetimeIndex(
              ["1990-01-01", "1990-01-02", "1990-01-03"],
              dtype="datetime64[ns]",
              freq="D",
          ),
      )
  
      assert (results[0] == expected[0]).all()
      assert (results[1] == expected[1]).all()
  
file_path: skforecast/skforecast/utils/tests/test_preproces_y.py
imports: import pytest
  import numpy
  import pandas
  from skforecast.utils import preprocess_y
label_desc: Use image node_id 1 for calling a global function with example usage: test_output_preprocess_y_when_y_index_is_DatetimeIndex_and_has_frequency() without return types