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

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

import socket
sock = socket.socket()
sock.bind(('', 0))
sock.getsockname()[1]

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().

You can listen on whatever port you want; generally, user applications should listen to ports 1024 and above (through 65535). The main thing if you have a variable number of listeners is to allocate a range to your app - say 20000-21000, and CATCH EXCEPTIONS. That is how you will know if a port is unusable (used by another process, in other words) on your computer.

However, in your case, you shouldn't have a problem using a single hard-coded port for your listener, as long as you print an error message if the bind fails.

Note also that most of your sockets (for the slaves) do not need to be explicitly bound to specific port numbers - only sockets that wait for incoming connections (like your master here) will need to be made a listener and bound to a port. If a port is not specified for a socket before it is used, the OS will assign a useable port to the socket. When the master wants to respond to a slave that sends it data, the address of the sender is accessible when the listener receives data.

I presume you will be using UDP for this?