code: def test_tutorial_downloadable():
      """
      Make sure that every tutorial that isn't in the whitelist has the placeholder
      that enables notebook download
      """
      download_button_string = "<!-- INSERT SOURCE DOWNLOAD BUTTONS -->"
  
      tutorial_path = os.path.join(
          os.path.dirname(__file__), "..", "..", "docs", "tutorials"
      )
      tutorials = glob.glob(os.path.join(tutorial_path, "**", "*.md"))
  
      for tutorial in tutorials:
          with open(tutorial, "r") as file:
              lines = file.readlines()
          last = lines[-1]
          second_last = lines[-2]
          downloadable = (
              download_button_string in last
              or download_button_string in second_last
          )
          friendly_name = "/".join(tutorial.split("/")[-2:])
          if not downloadable and friendly_name not in whitelist_set:
              print(last, second_last)
              assert (
                  False
              ), "{} is missing <!-- INSERT SOURCE DOWNLOAD BUTTONS --> as its last line".format(
                  friendly_name
              )
  
imports: import glob
  import os
  import re
label_desc: Use image node_id 1 for calling a global function with example usage: test_tutorial_downloadable() without return types