calendar-social/calsocial/calendar_system/__init__.py

class CalendarSystem:
    __has_months__ = False
    __has_weeks__ = False

    @property
    def day_names(self):
        raise NotImplementedError()

    @property
    def month(self):
        raise NotImplementedError()

    @property
    def days(self):
        raise NotImplementedError()