# tag::WilyDict[]
class WilyDict(dict):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__next_value = 0

    def __missing__(self, key):
        if key.startswith('__') and key.endswith('__'):
            raise KeyError(key)
        self[key] = value = self.__next_value
        self.__next_value += 1
        return value
# end::WilyDict[]

# tag::AUTOCONST[]
class AutoConstMeta(type):
    def __prepare__(name, bases, **kwargs):
        return WilyDict()

class AutoConst(metaclass=AutoConstMeta):
    pass
# end::AUTOCONST[]