<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def save(self, expires=None):
 """Save a copy of the object into the cache."""


        if expires is None:
            expires = self.expires
        s = self.serialize()
        key = self._key(self._all_keys())
        _cache.set(key, s, expires)