元编程-5

from collections import OrderedDict


class Typed:
    _expected_type = type(None)
    def __init__(self, name=None):
        self._name = name

    def __set__(self, instance, value):
        if not isinstance(value, self._expected_type):
            raise TypeError('Expected {}'.format(str(self._expected_type)))
        instance.__dict__[self._name] = value


class Integer(Typed):
    _expected_type = int


class Float(Typed):
    _expected_type = Float


class String(Typed):
    _expected_type = str


class OrderedMeta(type):
    def _ _new__(cls, clsname, bases, clsdict):
        d = dict(clsdict)
        order = []
        for name, value in clsdict.items():
            if isinstance(value, Typed):
                value._name = name
                order.append(name)
            d['_order'] = order
            return type.__new__(cls, clsname, bases, d)

    @classmethod
    def __prepare__(cls, clsname, bases):
        return OrderedDict()


class Structure(metaclass=OrderedMeta):
    def as_csv(self):
        return ','.join(str(getattr(self.name)) for name in self._order)


class Stock(Structure):
    name = String()
    shares = Integer()
    price = Float()

    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price