Missing recommended practice for replacing attribute with descriptor

[mwchase]

Blocked on python/mypy#4125

Motivation:

Dennis contains the following Coconut code

class Sprite:
    __slots__ = ()
    char: int
    name_: str
    color: colors.Color
    render_order: RenderOrder


data ConcreteSprite(char: int, name_: str, color: colors.Color, render_order: RenderOrder) from Sprite

class Alive(ConcreteSprite, enum.Enum):
    PLAYER = ConcreteSprite(ord("@"), "Player", colors.Colors.PLAYER, RenderOrder.ACTOR)
    ORC = ConcreteSprite(ord("o"), "Orc", colors.Colors.ORC, RenderOrder.ACTOR)
    TROLL = ConcreteSprite(ord("T"), "Troll", colors.Colors.TROLL, RenderOrder.ACTOR)


data Dead(corpse: Alive) from Sprite:
    char = ord("%")
    @property
    def name_(self) -> str:  # type: ignore
        corpse_name = self.corpse.name_
        if self.corpse != Alive.PLAYER:
            corpse_name = f"remains of {corpse_name}"
        return corpse_name
    color = colors.Colors.DEAD
    @property
    def render_order(self) -> RenderOrder:  # type: ignore
        if self.corpse == Alive.PLAYER:
            return RenderOrder.ACTOR
        return RenderOrder.CORPSE

Translated to pure-Python with Structured Data, it should be:

class Sprite:
    __slots__ = ()
    char: int
    name_: str
    color: colors.Color
    render_order: RenderOrder


class ConcreteSprite(Sprite, adt.Product):
    __slots__ = ()


# Can I switch this to just using Sprite?
# Might be blocked on https://github.com/mwchase/python-structured-data/issues/23
class Alive(ConcreteSprite, enum.Enum):
    PLAYER = ConcreteSprite(ord("@"), "Player", colors.Colors.PLAYER, RenderOrder.ACTOR)
    ORC = ConcreteSprite(ord("o"), "Orc", colors.Colors.ORC, RenderOrder.ACTOR)
    TROLL = ConcreteSprite(ord("T"), "Troll", colors.Colors.TROLL, RenderOrder.ACTOR)


class Dead(Sprite, adt.Product):
    corpse: Alive

    char: typing.ClassVar[int] = ord("%")
    name_: ???
    color: typing.ClassVar[colors.Color] = colors.Colors.DEAD
    render_order: ???

    @property
    def name_(self) -> str:
        corpse_name = self.corpse.name_
        if self.corpse != Alive.PLAYER:
            corpse_name = f"remains of {corpse_name}"
        return corpse_name

    @property
    def render_order(self) -> RenderOrder:
        if self.corpse == Alive.PLAYER:
            return RenderOrder.ACTOR
        return RenderOrder.CORPSE

(Maybe with some more __slots__ = () in there.)

[mwchase]

(Anyone wondering about the presence of both Color and Colors, that is unrelated to this, and something I intend to address at some point.)

[mwchase]

I did it.