code: def test_file_exists(self):
      # Make an empty file which has the same name as the cache directory
      with open(os.path.join(self.temp_dir, "_dl_cache"), "w"):
          pass
      with self.assertRaises(OSError):
          dataset.cached_download("https://example.com")
  
imports: import os
  import shutil
  import tempfile
  import unittest
  import mock
  from chainer import dataset
  from chainer import testing
label_desc: Use image node_id 4 for calling the TestCachedDownload obj's underlying member method code with example usage: obj.test_file_exists() without return types