> For reasoning about and maintaining, you can just think of `self` as an extra parameter to the function that gets bound to the thing to the left of the dot. In some languages, like Python, this is how it actually works, so the testing argument doesn't apply either:

In Python it isn't implicit, but it's still shared state, and so still hard to test and maintain. You can't test the `f` in your example without having to pass in the whole big bundle of state that self tends to be, even if you do so explicitly.