惰性属性

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @Time    : 2019-01-23 15:05
# @Author  : 
# @File    : lazyproperty.py
# @Software: Pycharm professional
import math


class lazyroperty:
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, cls):
        if instance is None:
            return self
        else:
            value = self.func(instance)
            setattr(instance, self.func.__name__, value)
            return value


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

    @lazyroperty
    def area(self):
        print("computing area")
        return math.pi * self.radius ** 2


if __name__ == '__main__':
    c = Circle(4.0)
    print(c.area)
    print(c.area)

可以看到上面代码的输出只有一遍"computing area"，惰性属性的全部意义就是用于提升程序性能，一旦访问了该属性就不会每次访问它的时候重新计算。

Python的hasattr() getattr() setattr() 函数使用方法详解：https://www.cnblogs.com/cenyu/p/5713686.html