class memoized(object):
   '''Decorator. Caches a function's return value each time it is called.
   If called later with the same arguments, the cached value is returned 
   (not reevaluated).
   '''
   def __init__(self, func):
       self.func = func
       self.cache = {}
   def __call__(self, *args):
       try:
           return self.cache[args]
       except KeyError:
           value = self.func(*args)
           self.cache[args] = value
           return value
       except TypeError:
           # uncachable -- for instance, passing a list as an argument.
           # Better to not cache than to blow up entirely.
           return self.func(*args)
   def __repr__(self):
       '''Return the function's docstring.'''
       return self.func.__doc__
   def __get__(self, obj, objtype):
      '''Support instance methods.'''
      return functools.partial(self.__call__, obj)