code: def test_union_over():
      """Test the vectorized function"""
      df = pd.DataFrame({"a": [], "aa": [], "b": [], "c": []})
  
      assert set(_with_prefix(df, ["a", "b"])) == {"a", "aa", "b"}
  
file_path: sklearn-evaluation/tests/test_DataSelector.py
imports: from pathlib import Path
  import pytest
  import numpy
  import pandas
  from sklearn_evaluation.training import DataSelector
  from sklearn_evaluation.training.selector import expand_arguments, _with_prefix
  from sklearn_evaluation.exceptions import DataSelectorError
label_desc: Use image node_id 3 for calling a global function with example usage: test_union_over() without return types