<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def authenticate(self, username, password):
 """ Authenticate user on server. :param username: Username used to be authenticated. :type username: six.string_types :param password: Password used to be authenticated. :type password: six.string_types :return: True if successful. :raises: InvalidCredentials, AuthenticationNotSupported, MemcachedException :rtype: bool """

        self._username = username
        self._password = password

        # Reopen the connection with the new credentials.
        self.disconnect()
        self._open_connection()
        return self.authenticated