> Hmm... OK, that's an interesting desire. How do square brackets get you any closer to that?
I use `__class_getitem__` as a memoized type factory that builds a new subtype when it's called or returns the cached type. You are correct I could name it as something else, there is nothing special about the square brackets other than notation but having a visual distinction of type creation and instantiation is useful. On Sat, Aug 15, 2020 at 3:44 PM David Mertz <me...@gnosis.cx> wrote: > On Sat, Aug 15, 2020 at 4:38 PM Caleb Donovick <donov...@cs.stanford.edu> > wrote: > >> > Why would it require a metaclass? Rather than just: ... >> >> Because I want the following to be true: >> x = Struct[x=int, y=str](...) >> assert isinstance(x, Struct) >> assert isinstance(x, Struct[x=int, y=str]) >> assert not isinstance(x, Struct[x=int, y=int]) >> > > Hmm... OK, that's an interesting desire. How do square brackets get you > any closer to that? > > If this proposal, in whatever variation, is adopted, `Struct[x=int, > y=str]` is going to be some kind of call to .__getitem__(). There is some > debate about exactly how the information gets passed into the method, but > we can bracket that for this question. One way or another, positional and > named arguments are available to this future .__getitem__(). > > So how do you make this true: > > assert isinstance(x, Struct.__getitem__(x=int, y=str)) > assert not isinstance(x, Struct.__getitem__(x=int, y=int)) > > For demonstration, maybe it's easiest just to give a new name to the > hypothetical method. Say `Struct.bracket(...)`. It's not obvious to me how > you'll get the behavior you want. > > ... and if you CAN get the behavior, why can't we name this method > .__call__()? > > I'm not really sure what kind of thing Struct is meant to be, as well. Is > it a class? An instance? A class factory? A metaclass? > > > -- > The dead increasingly dominate and strangle both the living and the > not-yet born. Vampiric capital and undead corporate persons abuse > the lives and control the thoughts of homo faber. Ideas, once born, > become abortifacients against new conceptions. >
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