class Circle(object): 
 def __init__(self, radius): 
  self.radius = radius 
  
 @property
 def area(self): 
  return 3.14 * self.radius ** 2
  
c = Circle(4) 
print c.radius 
print c.area

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