code: def ttl(self, key):
          
          key_value, key_age = self.get(key, with_age=True)
          if key_age:
              key_ttl = self.max_age - key_age
              if key_ttl > 0:
                  return key_ttl
          return None
docs: Return TTL of the `key` (in seconds).
  
          Returns None for non-existent or expired keys.