code: def mock_isfile(target_file, existing_files):
      if target_file in existing_files:
          return True
      return False
  
imports: import os
  import unittest
  from tempfile import NamedTemporaryFile
  import tabpy
  from tabpy.tabpy_server.app.util import validate_cert
  from tabpy.tabpy_server.app.app import TabPyApp
  from unittest.mock import patch
label_desc: Use image node_id 2 for calling the TestTransferProtocolValidation obj's underlying member method code with example usage: obj.mock_isfile(target_file, existing_files) and returns: False, True