On Fri, 20 Nov 2015 02:42 am, Ian Kelly wrote: > On Thu, Nov 19, 2015 at 5:45 AM, Steven D'Aprano <st...@pearwood.info> > wrote: >> But if you want the default value to be evaluated exactly once, and once >> only, there is no real alternative to early binding. You could use a >> global variable, of course, but that is no solution -- that's a problem >> waiting to happen. > > It doesn't have to be a global. In a Python with late binding > defaults, this would work to get early binding: > > def f(x=f.default_x): > ... > > f.default_x = something()
Ah, of course it can. Good catch. As an illustration of how your expectations can colour your thinking, I actually considered that, but rejected it as a possibility because f.default_x couldn't be created until the function came into existence, and the function couldn't come into existence until after f.default_x was evaluated. In other words, I was making the complementary error to BartC's: I was still thinking in terms of early binding, when describing late binding. In fact, what Python actually does is rather similar, except that instead of "f.default_x" it uses f.__defaults__[0]. Roughly speaking, the function declaration def f(x=something()): ... using early-binding is somewhat analogous to this using late-binding: def f(x=f.__defaults__[0]): ... f.__defaults__ = (something(), ) > Of course, assignment to function attributes is a relatively modern > thing that is only permitted since Python 2.1, whereas function > defaults have been around since at least 1.4, so this wouldn't have > been an option when the semantics were being determined. I suspect that had Guido decided on late binding as the preferred semantics, he would have allowed function attributes much earlier. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
