code: def test_column_ensemble_string_cols():
      """Check that ColumnEnsembleForecaster works with string columns."""
      df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
      fc = ColumnEnsembleForecaster(
          [(f"trans_{col}", NaiveForecaster(), col) for col in "ab"]
      )
      fc.fit(df, fh=[1, 42])
      fc.predict()
  
file_path: sktime/sktime/forecasting/compose/tests/test_column_ensemble.py
imports: import numpy
  import pandas
  import pytest
  from sktime.forecasting.compose import ColumnEnsembleForecaster
  from sktime.forecasting.naive import NaiveForecaster
  from sktime.forecasting.trend import PolynomialTrendForecaster
  from sktime.utils.validation._dependencies import _check_soft_dependencies
label_desc: Use image node_id 3 for calling a global function with example usage: test_column_ensemble_string_cols() without return types