Yeah, that's more or less what happens. What you'd, admittedly naively, want, though, is a way of saying "a new empty dictionary, please", so you don't wind up

Yeah, that's more or less what happens. What you'd, admittedly naively, want, though, is a way of saying "a new empty dictionary, please", so you don't wind up having to write this kind of thing: def f(a, d=None): if d is None: d = {} which, okay, it's a minor deal, and I know why it happens, but it's the kind of thing I forget you need to do!