>>> cache = Cache(log_level=logging.WARNING)
        >>> def cache_loader(key): pass
        >>> cache.with_cache_loader(cache_loader)
        True
        >>> def cache_loader_b(key, value): pass
        >>> cache.with_cache_loader(cache_loader_b)
        False

def with_cache_loader(self, func, only_read=False):
        """
        >>> cache = Cache(log_level=logging.WARNING)
        >>> def cache_loader(key): pass
        >>> cache.with_cache_loader(cache_loader)
        True
        >>> def cache_loader_b(key, value): pass
        >>> cache.with_cache_loader(cache_loader_b)
        False
        """
        self.logger.info('Enabled cache loader %s' % get_function_signature(func))
        self.cache_loader = func
        return True