utils.py

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):
       if not isinstance(args, collections.Hashable):
           # uncacheable. a list, for instance.
           # better to not cache than blow up.
           return self.func(*args)
       if args in self.cache:
           return self.cache[args]
       else:
           value = self.func(*args)
           self.cache[args] = value
           return value

    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)