Ndtyjky

I am defining a Python Holiday class and three subclasses: one for fixed date holidays, a second for relative holidays, and a third for floating Monday holidays. I would like to create a set of constants in the superclass Holiday so that applications can simply refer to particular holidays as

Holiday.NEW_YEARS

Holiday.CHRISTMAS

etc.

but the subclasses obviously do not exist when the parent class is instantiated. How can I do this?

class Holiday(object):

    NEW_YEARS = FixedHoliday(1, 1)

    MLK_BIRTHDAY = FloatingMonday(1, 15)

    ...

A class can't and shouldn't refer to its derived classes. Try this instead:

class BaseHoliday(object):

    pass



class FixedHoliday(BaseHoliday):

    # class code



# more classes



class Holidays(object):

    NEW_YEARS = FixedHoliday(1, 1)

    MLK_BIRTHDAY = FloatingMonday(1, 15)

One way to do it would be to decorate the base class after it and all the subclasses are defined. You can this in Python because classes are mutable objects.

Here's what I'm suggesting:

class Holiday:

    def __init__(self, month, day):

        self.month, self.day = month, day



    def __repr__(self):

        return '{}(month={}, day={})'.format(type(self).__name__, self.month, self.day)



class FixedHoliday(Holiday):

    pass



class FloatingMonday(Holiday):

    pass



#class MondayHoliday(Holiday): ... etc    





# Base class decorator.

def inject_constants(cls):

    """ Add attributes to class. """

    HOLIDATA = {

        'NEW_YEARS': FixedHoliday(1, 1),

        'MLK_BIRTHDAY': FloatingMonday(1, 15)

    }



    for key, value in HOLIDATA.items():

        setattr(cls, key, value)





inject_constants(Holiday)



print(Holiday.NEW_YEARS)    # -> FixedHoliday(month=1, day=1)

print(Holiday.MLK_BIRTHDAY) # -> FloatingMonday(month=1, day=15)