code: def test_single_file(self):
      # no suffix
      result = filter_files(pj(DATA_PATH, "README"), None)
      self.assertEqual(result, [pj(DATA_PATH, "README")])
      # single suffix
      result = filter_files(pj(DATA_PATH, "README"), suffix=".txt")
      self.assertEqual(result, [])
      # suffix list
      result = filter_files(
          pj(DATA_PATH, "events.txt"), suffix=[".txt", None]
      )
      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 1 for calling the TestFilterFilesFunction obj's underlying member method code with example usage: obj.test_single_file() without return types