def conchTestPublicKeyChecker():
        """
        Produces a SSHPublicKeyChecker with an in-memory key mapping with
        a single use: 'testuser'

        @return: L{twisted.conch.checkers.SSHPublicKeyChecker}
        """
        conchTestPublicKeyDB = checkers.InMemorySSHKeyDB(
            {b'testuser': [keys.Key.fromString(publicDSA_openssh)]})
        return checkers.SSHPublicKeyChecker(conchTestPublicKeyDB)

        Produces a SSHPublicKeyChecker with an in-memory key mapping with
        a single use: 'testuser'

        @return: L{twisted.conch.checkers.SSHPublicKeyChecker}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_ssh.py