code: def test_get_feature_names_out_single_variable_and_single_function(
      df_time,
  ):
      # input features
      original_features = [
          "ambient_temp",
          "module_temp",
          "irradiation",
          "color",
      ]
  
      tr = ExpandingWindowFeatures(
          variables="ambient_temp", functions="sum"
      )
      tr.fit(df_time)
  
      # expected
      output = [
          "ambient_temp_expanding_sum",
      ]
      output = original_features + output
      assert tr.get_feature_names_out(input_features=None) == output
  
file_path: feature_engine/tests/test_time_series/test_forecasting/test_expanding_window_features.py
imports: import numpy
  import pytest
  from pandas.testing import assert_frame_equal
  from feature_engine.timeseries.forecasting import ExpandingWindowFeatures
label_desc: Use image node_id 6 for calling a global function with example usage: test_get_feature_names_out_single_variable_and_single_function(df_time) without return types