Even in the case you mention you really shouldn't be overriding these methods. Your load settings method should take the path of the settings file as an argument, and then your test can set up all the fake files you want with something like python's tempfile package

There are a number of different ways to solve this problem. I too use the path of settings in my code, but I'm not against overriding open and all the other file io functions. Of course this article is about python which has different abilities than other languages, what is best in python is not what is best in other languages, and I'm trying to stay at a higher level that a particular language.