lazy property

lazyproperty.py

import math


class lazyproperty(object):
    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(object):
    def __init__(self, radius):
        self.radius = radius

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

if __name__ == '__main__':
    c = Circle(4.0)

    # lazyproperty calls setattr(instance, self.func.__name__, value)
    print c.area

    # just get previously set property
    print c.area