Python クラス属性の遅延評価

class LazyProperty(object):
    def __init__(self, func):
        self.func = func    def __get__(self, instance, owner):
        if instance is None:            return self        else:
            value = self.func(instance)
            setattr(instance, self.func.__name__, value)            return valueimport mathclass Circle(object):
    def __init__(self, radius):
        self.radius = radius    @LazyProperty
    def area(self):
        print 'Computing area'
        return math.pi * self.radius ** 2    @LazyProperty
    def perimeter(self):
        print 'Computing perimeter'
        return 2 * math.pi * self.radius

>>> c = Circle(2)
>>> print c.area
Computing area
12.5663706144
>>> print c.area
12.5663706144