Python 描述符的高级用法

class Type:
    def __init__(self, name, expected_type):
        self.name = name
        self.expected_type = expected_type

    def __get__(self, instance, owner):
        if instance is None:
            return self
        else:
            return instance.__dict__[self.name]

    def __set__(self, instance, value):

        print(self.expected_type)
        print(value)
        if not isinstance(value, self.expected_type):
            raise TypeError('Expected ' + str(self.expected_type))
        instance.__dict__[self.name] = value

    def __delete__(self, instance):
        del instance.__dict__[self.name]


def type_assert(**kwargs):
    def decorate(cls):
        for name, expected_type in kwargs.items():
            setattr(cls, name, Type(name, expected_type))
        return cls

    return decorate


@type_assert(name=str, shares=int, price=float)
class Stock:
    def __init__(self, name, shares, price):
        self.name = name
        self.shares = shares
        self.price = price

    def __call__(self, *args, **kwargs):
        return args


if __name__ == '__main__':
    s = Stock('wsg', 10, 2.3)

另一种类型

class Descriptor:
    def __init__(self, name=None, **kwargs):
        self.name = name
        for key, value in kwargs.items():
            setattr(self, key, value)

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value


class Typed(Descriptor):
    expected_type = type(None)

    def __set__(self, instance, value):
        if not isinstance(value, self.expected_type):
            raise TypeError('Excepted ' + str(self.expected_type))
        super().__set__(instance, value)


class Unsigned(Descriptor):
    def __set__(self, instance, value):
        if value < 0:
            raise ValueError('Expected >=0 ')
        super().__set__(instance, value)


class MaxSized(Descriptor):
    def __init__(self, name=None, **kwargs):
        if 'size' not in kwargs:
            raise TypeError('missing size option')
        super().__init__(name, **kwargs)

    def __set__(self, instance, value):
        if len(value) >= self.size:
            raise ValueError('size must be < ' + str(self.size))
        super().__set__(instance, value)


class Integer(Typed):
    expected_type = int


class UnsignedInteger(Integer, Unsigned):
    pass


class Float(Typed):
    expected_type = float


class UnsignedFloat(Float, Unsigned):
    pass


class String(Typed):
    expected_type = str


class SizedString(String, MaxSized):
    pass


class Stock:
    name = SizedString('name', size=8)
    shares = UnsignedInteger('shares')
    price = UnsignedFloat('price')

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


s = Stock('morgan', 10, 6.99)
print(vars(s))

可以用元类的方式，简化上面的代码

class CheckMate:
    def __new__(cls,clsname,bases,methods):
        for key,value in methods.items():
            if isinstance(value,Descriptor):
                value.name = key
        return type.__new__(cls,clsname,bases,methods)
class Stock2(mateclass=CheckMate):
    name = SizedString(size=8)
    shares = UnsignedInteger()
    price = UnsignedFloat()
    def __init__(self,name,shares,price):
        self.name = name
        self.shares = shares
        self.price = price
s2 = Stock2('morgan2',10,66.66)
print(vars(s2))