what is mean about isinstance(msg, tuple)?

[catroll]

            try:
                server.send_cmds(b''.join(bigcmd))
            except socket.error as msg:
                rc = 0
                if isinstance(msg, tuple):
                    msg = msg[1]
                server.mark_dead(msg)
                dead_servers.append(server)

why a exception may be a tuple?