On localhost, how do I pick a free port number?

Question

Do not bind to a specific port. Instead, bind to port 0:

sock.bind(('', 0))

The OS will then pick an available port for you. You can get the port that was chosen using sock.getsockname()[1], and pass it on to the slaves so that they can connect back.

sock is the socket that you created, returned by socket.socket.

For the sake of snippet of what the guys have explained above:

import socket
from contextlib import closing

def find_free_port():
    with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
        s.bind(('', 0))
        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        return s.getsockname()[1]

Bind the socket to port 0. A random free port from 1024 to 65535 will be selected. You may retrieve the selected port with getsockname() right after bind().

If you only need to find a free port for later use, here is a snippet similar to a previous answer, but shorter, using socketserver:

import socketserver

with socketserver.TCPServer(("localhost", 0), None) as s:
    free_port = s.server_address[1]

Note that the port is not guaranteed to remain free, so you may need to put this snippet and the code using it in a loop.