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 value


import math


class 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