code: def test_file_download_happens_once(self):
      # Tests that a file is only downloaded once if it's not updated.
      with SoftTemporaryDirectory() as cache:
          path = hf_hub_download(
              MODEL_IDENTIFIER, "file_0.txt", cache_dir=cache
          )
          creation_time_0 = os.path.getmtime(path)
  
          time.sleep(2)
  
          path = hf_hub_download(
              MODEL_IDENTIFIER, "file_0.txt", cache_dir=cache
          )
          creation_time_1 = os.path.getmtime(path)
  
          self.assertEqual(creation_time_0, creation_time_1)
  
imports: import os
  import time
  import unittest
  from io import BytesIO
  from huggingface_hub import HfApi, hf_hub_download, snapshot_download
  from huggingface_hub.utils import SoftTemporaryDirectory, logging
  from huggingface_hub.utils._errors import EntryNotFoundError
  from .testing_constants import ENDPOINT_STAGING, TOKEN
  from .testing_utils import repo_name, with_production_testing, xfail_on_windows
label_desc: Use image node_id 3 for calling the CacheFileLayoutHfHubDownload obj's underlying member method code with example usage: obj.test_file_download_happens_once() without return types