code: def test_clearsky_trafo_nofreq_exception():
      """Tests clear sky trafo exception with no set/inferable freq in index."""
      y = load_solar(api_version=None)
  
      # no set or inferable frequency should not work
      y = y.drop(pd.to_datetime("2021-05-01 00:30:00", utc=True))
      cs_model = ClearSky()
      with pytest.raises(ValueError):
          cs_model.fit_transform(y)
  
file_path: sktime/sktime/transformations/series/tests/test_clear_sky.py
imports: import numpy
  import pandas
  import pytest
  from sktime.datasets import load_solar
  from sktime.tests.test_switch import run_test_for_class
  from sktime.transformations.series.clear_sky import ClearSky
label_desc: Use image node_id 3 for calling a global function with example usage: test_clearsky_trafo_nofreq_exception() without return types