code: def test_expanding_sum_single_var_freqs(df_time):
      expected_results = {
          "ambient_temp_expanding_sum": [
              np.nan,
              np.nan,
              31.31,
              62.82,
              94.97,
              127.36,
              159.98,
              192.48,
              225.00,
              257.68,
              291.44,
              325.57,
              359.65,
              393.35,
              427.24,
          ],
      }
      expected_df = df_time.copy()
      expected_df["ambient_temp_expanding_sum"] = expected_results[
          "ambient_temp_expanding_sum"
      ]
  
      transformer = ExpandingWindowFeatures(
          variables=["ambient_temp"], functions="sum", freq="30T"
      )
      df_tr = transformer.fit_transform(df_time)
      assert_frame_equal(df_tr, expected_df)
  
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 12 for calling a global function with example usage: test_expanding_sum_single_var_freqs(df_time) without return types