text: """Tests for Hook module."""
  
  import pytest
  import unittest
  
  from toolkit.transformers import Hook
  
  
  class TestHook(unittest.TestCase):
      """Tests for Hook class."""
  
      def test_hook(self):
          """Test Hook initialization and key handling."""
          hook = Hook(key='key', func=lambda: 'test')
  
          self.assertEqual(hook.key, 'key')
          # w/o args
          self.assertEqual(hook(), 'test')
          # check invalid key
          with pytest.raises(ValueError):
              _ = Hook(key='key', func=lambda: None)
  
          # hook with args
          hook_args = Hook(key='key_', func=lambda x: x)
  
          self.assertIsNotNone(hook_args)
          self.assertEqual(Hook.get_current_keys(), {'key', 'key_'})
          self.assertEqual(hook_args.key, 'key_')
  