class DataSetCache(object):
    def __init__(self):
        super(Cache, self).__init__()
        self._cache = dict()
        self._generator = dict()
    def __call__(self, generator):
        name = generator.func_name
        self._cache[name] = dict()
        self._generator[name] = generator
        def gen(date):
            if not self._cache[name].has_key(date):
                self._cache[name][date] = self._generator[name](date)
            return self._cache[name][date]
        gen.func_name = name
        return gen
cache = DataSetCache()

@cache