choose a port number unused for TCP on all local inet interfaces (or on a particular interface). use sock_type=socket.SOCK_DGRAM for an unused UDP port instead of TCP.

unused_port.py

import socket
import contextlib

def choose_unused_port(interface='0.0.0.0', sock_type=socket.SOCK_STREAM):
    with contextlib.closing(socket.socket(socket.AF_INET, sock_type)) as s:
        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        s.bind((interface, 0))
        return s.getsockname()[1]