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witty-crayon-22786

07/22/2020, 6:41 PM
@hundreds-father-404: random, but while looking at https://github.com/pantsbuild/pants/issues/9760 , i realized that in many cases the “memoized anonymous class” strategy can typecheck fine… the only time it won’t is when the subclass needs to be treated as the subclass (that would require casting). otherwise you can type it like:
Copy code
class SS:
    @memoized
    @classmethod
    def scoped_to(cls, name: str) -> Type["SS"]:
        class AnonymousSub(SS):
            pass
        return AnonymousSub
ie, if you are ok with losing the precise type in mypy, you’re good. it’s still a distinct, unique subclass at runtime… which is sufficient for unions/the-engine
so, that means it would work for 
Target.Fields
, and for the usecase on #9760.
but it did not work for 
Options
(afaik) because we wanted to preserve the subtype to be able to call methods on it and/or see which options were valid
(although as i say that, i’m not sure it’s true, because Options instances don’t have any unique fields, so you never need the more precise type)
h

hundreds-father-404

07/22/2020, 6:44 PM
Target.Fields
? Like union fields?
w

witty-crayon-22786

07/22/2020, 6:44 PM
yea, i forget where it is
h

hundreds-father-404

07/22/2020, 6:45 PM
Oh, I think you mean 
.PluginField
, which John found is the same for every target type
w

witty-crayon-22786

07/22/2020, 6:46 PM
right. so: it doesn’t need to be if you use the anonymous class with memoization.
it can in fact be a distinct type: see the example above
👀 1
h

hundreds-father-404

07/22/2020, 6:47 PM
I’m confused why 
AnonymousSub
subclasses the superclass. Is that necessary?
w

witty-crayon-22786

07/22/2020, 6:51 PM
yes: to typecheck
it is a subclass of the type you’re returning
you’re not declaring its precise type: you’re just saying that you’re returning a subtype of 
SS
👍 1
cc @enough-analyst-54434: ^ fyi
e

enough-analyst-54434

07/27/2020, 5:05 PM
K. I still think modeling fields you add to a target as subclasses of some marker field type + warping Union is all tortured vs directly lifting this as 1st class concept - just register the plugin field directly, but Its already modeled that way though, so this does no extra harm and rights the ship at least.
👍 1
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