Question

我有这个

#!/usr/bin/env python

import math
class myclass(object):

    def __init__(self,radius):
            self.radius = radius
    @property
    def area(self):
            print myclass.area,  type(myclass.area)
            return math.pi * ( self.radius ** 2)

    @area.setter
    def area(self,value):
           print myclass.area, type(myclass.area)
           pass

    @area.deleter
    def area(self):
           print myclass.area, type(myclass.area)        
           del myclass.area                


if __name__ == '__main__':

    c = myclass(5.4)
    c.area
    c.area = 65
    del c.area

这给出了：

$ ./propertytest.py 
<property object at 0x7ff0426ac0a8> <type 'property'>
<property object at 0x7ff0426ac0a8> <type 'property'>
<property object at 0x7ff0426ac0a8> <type 'property'>

问题：

查看已访问属性对象area的方式：c.area。 area出现在点运算符的右侧。属性对象使用哪种特殊方法将类实例对象与正确的实例方法绑定并计算结果？物业如何运作？

Answer 1

属性是描述符。描述符是具有__get__，__set__和/或__delete__方法的类的实例。每当python执行getattr，setattr和delattr时，如果实例没有所述属性，并且在类中有一个具有该名称的属性，它具有匹配的魔术方法，调用magic方法而不是读/写/删除属性。

Python documentation详细介绍了描述符，并且还有内置property类型的纯python仿真：

class Property(object):
    "Emulate PyProperty_Type() in Objects/descrobject.c"

    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj)

    def __set__(self, obj, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(obj, value)

    def __delete__(self, obj):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(obj)

    def getter(self, fget):
        return type(self)(fget, self.fset, self.fdel, self.__doc__)

    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.__doc__)

    def deleter(self, fdel):
        return type(self)(self.fget, self.fset, fdel, self.__doc__)

Answer 2

为了使上面发生的事情更加明确，让我们摆脱装饰器语法糖：

import math
class myclass(object):

    def __init__(self,radius):
            self.radius = radius

    def getter(self):
            print myclass.area,  type(myclass.area)
            return math.pi * ( self.radius ** 2)

    # create a property object with "getter" as the getter, assigned to "area"
    area = property(getter) 

    # area.__get__ is now a method that wraps "getter"



    def setter(self,value):
           print myclass.area, type(myclass.area)
           pass

    # create a copy of the "area" object with "setter" as a setter, assign back to "area"
    area = area.setter(setter) 

    # area.__set__ is now a method that wraps "setter"



    def deleter(self):
           print myclass.area, type(myclass.area)        
           del myclass.area  

    # create a copy of the "area" object with "deleter" as a deleter, assign back to "area"
    area = area.deleter(deleter)

    # area.__delete__ is now a method that wraps "deleter"

python属性类实例如何工作？

2 个答案: