model.py

class Model(object):
    accessors = {}

    def __init__(self, hash = None, options = {}):
        self._dynamic_accessors = []
        if hash:
            self.__class__.mass_assign(self, hash, {})

    @classmethod
    def add_field(cls, fname, type_f, options={}):
        setattr(cls, fname, None)
        if not cls.__class__.accessors:
            cls.__class__.accessors[cls.__class__] = []
        exclude = options.get('exclude', [])
        cls.__class__.accessors[cls.__class__].append({'name': fname, 'type': type_f, 'exclude': exclude})