Caching decorator (AOP?!)

rpc.py

def cached(key_formatstr):
    def decorator(fn):
        def wrapped(self, *args, **kwargs):
            cache_key = key_formatstr.format(*args, **kwargs)
            log.debug('Fetching cached value for {0}'.format(cache_key))
            value = self.mc_get(cache_key)
            if not value:
                value = fn(self, *args, **kwargs)
                log.debug('Storing cached value for {0}'.format(cache_key))
                self.mc_set(cache_key, value)
            return value
        return wrapped
    return decorator

class MyRPC:
    @cached("user:{0}")
    def get_user(user_id):
        return requests.get("http://rest.myapi.com/user/{0}".format(user_id)