original_save = Item.save

def side_fx(self, *a, **kw):
    side_fx.self = self
return original_save(self, *a, **kw)
with patch.object(Item, 'save', autospec=True, side_effect=side_fx
    ) as mock_save:
    data = 'the data'
instance = SomeClass()
instance.some_method(data)
mock_save.assert_called_once_with(side_fx.self, data)
How to call self in a mock method of an object in Python?