code: def test_sample_columns():
      df = pd.DataFrame(
          columns=["feature1", "feature2", "feature3", "feature4"]
      )
      expected_len = 2
      found = sample_columns(data=df, perc=0.5)
      assert expected_len == len(found)
      assert all([el in df.columns for el in found])
  
file_path: fklearn/tests/validation/test_perturbators.py
imports: from fklearn.validation.perturbators import shift_mu, random_noise, nullify, sample_columns, perturbator
  import pandas
  import numpy
label_desc: Use image node_id 4 for calling a global function with example usage: test_sample_columns() without return types