code: def test_file_list(self):
      # no suffix
      result = filter_files(FILE_LIST, None)
      self.assertEqual(result, FILE_LIST)
      # single suffix
      result = filter_files(FILE_LIST, suffix="txt")
      self.assertEqual(
          result,
          [pj(DATA_PATH, "commented_txt"), pj(DATA_PATH, "events.txt")],
      )
      # suffix list
      result = filter_files(FILE_LIST, suffix=[".txt"])
      self.assertEqual(result, [pj(DATA_PATH, "events.txt")])
  
imports: from __future__ import absolute_import, division, print_function
  import unittest
  from os.path import join
  from madmom.utils import *
  from .None import ACTIVATIONS_PATH, ANNOTATIONS_PATH, AUDIO_PATH, DATA_PATH, DETECTIONS_PATH
  from .test_evaluation_notes import ANNOTATIONS
label_desc: Use image node_id 2 for calling the TestFilterFilesFunction obj's underlying member method code with example usage: obj.test_file_list() without return types