Sync the write buffer, then close the cache.

        If a closed :class:`FileCache` object's methods are called, a
        :exc:`ValueError` will be raised.

def close(self):
        """Sync the write buffer, then close the cache.

        If a closed :class:`FileCache` object's methods are called, a
        :exc:`ValueError` will be raised.

        """
        self.sync()
        self.sync = self.create = self.delete = self._closed
        self._write_to_file = self._read_to_file = self._closed
        self._key_to_filename = self._filename_to_key = self._closed
        self.__getitem__ = self.__setitem__ = self.__delitem__ = self._closed
        self.__iter__ = self.__len__ = self.__contains__ = self._closed