human
Solve the following python programming problem given the following code:
@ mock.patch.object (lockutils, 'external_lock') 
@ mock.patch.object (libvirt_utils, 'update_mtime') 
@ mock.patch ('os.path.exists', return_value = True) 
@ mock.patch ('os.path.getmtime') 
@ mock.patch ('os.remove') 
def test_age_and_verify_swap_images(self, mock_remove, mock_getmtime, mock_exist, mock_mtime, mock_lock) : 
    image_cache_manager = imagecache.ImageCacheManager () 
    expected_remove = set () 
    expected_exist = set (['swap_128', 'swap_256']) 
    image_cache_manager.back_swap_images.add ('swap_128') 
    image_cache_manager.back_swap_images.add ('swap_256') 
    image_cache_manager.used_swap_images.add ('swap_128') 
    def getmtime(path) : 
        return (time.time () - 1000000) 
mock_getmtime.side_effect = getmtime 
    def removefile(path) : 
        if (not path.startswith ('/tmp_age_test')) : 
            return os.remove (path) 
fn = os.path.split (path) [(- 1)] 
        expected_remove.add (fn) 
        expected_exist.remove (fn) 
mock_remove.side_effect = removefile 
    image_cache_manager._age_and_verify_swap_images (None, '/tmp_age_test') 
    self.assertEqual (1, len (expected_exist)) 
    self.assertEqual (1, len (self)) 
    self.assertIn ('swap_128', expected_exist) 
    self.assertIn ('swap_256', expected_remove) 

    
    

    Fix the buggy line: self.assertEqual (1, len (self))
assistant
#Here is Python code to fix the buggy line: self.

self.assertEqual (1, len (expected_remove))