def requestAvatar(self, avatarId, mind, *interfaces):
        """
        Retrieve a mailbox for I{testuser} or fail.

        @param avatarId: See L{IRealm.requestAvatar}.
        @param mind: See L{IRealm.requestAvatar}.
        @param interfaces: See L{IRealm.requestAvatar}.

        @raises: L{AssertionError} when requesting an C{avatarId} other than
            I{testuser}.
        """
        if avatarId == b'testuser':
            return pop3.IMailbox, DummyMailbox(ValueError), lambda: None
        assert False

        Retrieve a mailbox for I{testuser} or fail.

        @param avatarId: See L{IRealm.requestAvatar}.
        @param mind: See L{IRealm.requestAvatar}.
        @param interfaces: See L{IRealm.requestAvatar}.

        @raises: L{AssertionError} when requesting an C{avatarId} other than
            I{testuser}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_pop3.py