code: def test_multiplex_or_dunder():
      """Test that the MultiplexForecaster magic "|" dunder methodbahves as expected.
  
      A MultiplexForecaster can be created by using the "|" dunder method on either
      forecaster or MultiplexForecaster objects. Here we test that it performs as expected
      on all the use cases, and raises the expected error in some others.
      """
      # test a simple | example with two forecasters:
      multiplex_two_forecaster = AutoETS() | NaiveForecaster()
      assert isinstance(multiplex_two_forecaster, MultiplexForecaster)
      assert len(multiplex_two_forecaster.forecasters) == 2
      # now test that | also works on two MultiplexForecasters:
      multiplex_one = MultiplexForecaster(
          [("arima", AutoARIMA()), ("ets", AutoETS())]
      )
      multiplex_two = MultiplexForecaster(
          [("theta", ThetaForecaster()), ("naive", NaiveForecaster())]
      )
      multiplex_two_multiplex = multiplex_one | multiplex_two
      assert isinstance(multiplex_two_multiplex, MultiplexForecaster)
      assert len(multiplex_two_multiplex.forecasters) == 4
      # last we will check 3 forecaster with the same name - should check both that
      # MultiplexForecaster | forecaster works, and that ensure_unique_names works
      multiplex_same_name_three_test = (
          NaiveForecaster(strategy="last")
          | NaiveForecaster(strategy="mean")
          | NaiveForecaster(strategy="drift")
      )
      assert isinstance(
          multiplex_same_name_three_test, MultiplexForecaster
      )
      assert len(multiplex_same_name_three_test.forecasters) == 3
      forecaster_param_names = (
          multiplex_same_name_three_test._get_estimator_names(
              multiplex_same_name_three_test._forecasters
          )
      )
      assert len(set(forecaster_param_names)) == 3
  
      # test we get a ValueError if we try to | with anything else:
      with pytest.raises(TypeError):
          multiplex_one | "this shouldn't work"
  
file_path: sktime/sktime/forecasting/compose/tests/test_multiplex.py
imports: import pytest
  from sktime.datasets import load_shampoo_sales
  from sktime.forecasting.arima import AutoARIMA
  from sktime.forecasting.compose import MultiplexForecaster
  from sktime.forecasting.ets import AutoETS
  from sktime.forecasting.model_evaluation import evaluate
  from sktime.forecasting.model_selection import ForecastingGridSearchCV
  from sktime.forecasting.naive import NaiveForecaster
  from sktime.forecasting.theta import ThetaForecaster
  from sktime.split import ExpandingWindowSplitter
  from sktime.utils.validation._dependencies import _check_estimator_deps
  from sktime.utils.validation.forecasting import check_scoring
label_desc: Use image node_id 4 for calling a global function with example usage: test_multiplex_or_dunder() without return types