choices.py

from collections import OrderedDict

empty = object()


class Item:
    """
    Descriptor
    Usage:
     >>>    class SomethingType:
     >>>        FIRST_TYPE = Item()
     >>>        SECOND_TYPE = Item('special')
     >>>    assert SomethingType.FIRST_TYPE == 'first_type'
     >>>    assert SomethingType.SECOND_TYPE == 'special'
    """

    def __init__(self, value: str = empty):
        self._value = value

    def __get__(self, instance, owner):
        return self._value

    def __set__(self, instance, value):
        self._value = value

    def __set_name__(self, owner, name):
        if not name.isupper():
            raise NameError(
                f'{owner.__qualname__}.{name} must be in uppercase'
            )
        if self._value is empty:
            self._value = name.lower()


class ChoiceMeta(type):
    _items: OrderedDict

    def __new__(mcs, name, bases, attrs):
        attrs['_items'] = mcs._get_items(bases, attrs)
        cls = super(ChoiceMeta, mcs).__new__(mcs, name, bases, attrs)
        return cls

    @classmethod
    def _get_items(mcs, bases, attrs):
        items = [
            (name, item)
            for name, item in attrs.items()
            if isinstance(item, Item)
        ]

        known = set(attrs)

        base_items = [
            (known.add(name) or name, item)
            for base in bases if hasattr(base, '_items')
            for name, item in base._items.items() if name not in known
        ]

        return OrderedDict(base_items + items)

    def __iter__(cls):
        for k, v in cls._items.items():
            if isinstance(v, Item):
                yield (k, getattr(cls, k))

    @property
    def choices(cls):
        return list(iter(cls))


class Choice(metaclass=ChoiceMeta):
    @property
    def choices(self):
        return self.__class__.choices


if __name__ == '__main__':
    class BaseChoice(Choice):
        BASE = Item('special')

    class ModeChoice(BaseChoice):
        ADDRESS = Item()
        KEY = Item()

    print(BaseChoice.choices)
    print(BaseChoice().choices)

    print(BaseChoice.BASE)
    print(BaseChoice().BASE)

    print(ModeChoice.choices)
    print(ModeChoice().choices)

    print(ModeChoice.BASE)
    print(ModeChoice.ADDRESS)
    print(ModeChoice.KEY)