example of chaining and class attribute usage like drf

chain_and_class_attr.py

import functools


class Field(object):
    def __init__(self, *args, **kwargs):
        pass


class ChainAbleMixin(object):
    def __getattr__(self, item):
        if item != 'then':
            raise AttributeError(item)
        return self.chain

    def chain(self, *args, **kwargs):
        cls = args[0]
        setattr(self, 'next', cls)
        return self


class ReduceFrom(ChainAbleMixin, Field):
    def __init__(self, *args, **kwargs):
        super(ReduceFrom, self).__init__(*args, **kwargs)
        self.lookup = args[0]

    def get_value(self):
        return functools.reduce(dict.get, self.lookup, self.context)


class CastInto(ChainAbleMixin, Field):
    def __init__(self, *args, **kwargs):
        super(CastInto, self).__init__(*args, **kwargs)
        self.type = args[0]

    def get_value(self):
        return self.type(self.context)


class Mapping(Field):
    _skip_fields = ['process', 'chain']

    def process(self):
        fields = [field for field in dir(self) if not field.startswith('_')
                  and field not in self._skip_fields]

        for field in fields:
            cls = getattr(self, field)
            setattr(cls, 'context', self._data)
            while hasattr(cls, 'next'):
                value = cls.get_value()
                cls = getattr(cls, 'next')
                setattr(cls, 'context', value)
            value = cls.get_value()
            setattr(self, field, value)


class PriceMapping(Mapping):
    price = ReduceFrom(['paraparapara']).then(CastInto(int))

a = PriceMapping()
a._data = {'paraparapara': '11'}
a.process()