human
Solve the following python programming problem given the following code:
def _test_create_image(self, cache_type) : 
    instance = { 
        'system_metadata' : { 
            'image_cache_in_nova' : True, 
}, 
} 
    self.flags (cache_images = cache_type, group = 'xenserver') 
    was = { 
        'called' : None, 
} 
    def fake_create_cached_image(* args) : 
        was ['called'] = 'some' 
        return (False, { 
            
}) 
self.stubs.Set (vm_utils, '_create_cached_image', <MASK>) 
    def fake_fetch_image(* args) : 
        was ['called'] = 'none' 
        return { 
            
} 
self.stubs.Set (vm_utils, '_fetch_image', fake_fetch_image) 
    vm_utils.create_image (self.context, None, instance, 'foo', 'bar', 'baz') 
    self.assertEqual (was ['called'], cache_type) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: was, self, fake_fetch_image, cache_type, instance
assistant
#Here is a proper replacement for <MASK>.
def fake_create_cached_image(