code: def with_cache_writer(self, func, only_read=False):
          
          self.logger.info('Enabled cache writer %s' % get_function_signature(func))
          self.cache_writer = func
          return True
docs: >>> cache = Cache(log_level=logging.WARNING)
          >>> def cache_writer(key): pass
          >>> cache.with_cache_writer(cache_writer)
          False
          >>> def cache_writer(key, value): pass
          >>> cache.with_cache_writer(cache_writer)
          True