code: def test_output_expand_index_when_index_is_DatetimeIndex():
      """
      Test values returned by expand_index when input is DatetimeIndex.
      """
      index = pd.DatetimeIndex(
          ["1990-01-01", "1990-01-02", "1990-01-03"],
          dtype="datetime64[ns]",
          freq="D",
      )
      expected = pd.DatetimeIndex(
          ["1990-01-04", "1990-01-05", "1990-01-06"],
          dtype="datetime64[ns]",
          freq="D",
      )
      results = expand_index(index, steps=3)
  
      assert (results == expected).all()
  
imports: import pytest
  import numpy
  import pandas
  from skforecast.utils import expand_index
label_desc: Use image node_id 1 for calling a global function with example usage: test_output_expand_index_when_index_is_DatetimeIndex() without return types