That python code does not specify the port to open on the client. If it did, there would have been a third argument to socket.socket. Leaving out the third argument, which is the port number on the client, is the equivalent to passing 0 as the port to :gen_udp.open.

Only root is allowed to bind to ports less than 1024.

The port parameter in :gen_udp.open/1 is the local port on which the application receives datagrams. If you’re implementing a client you don’t need to bind to port 123, you can just let the OS pick an available (unprivileged) port: :gen_udp.open(0).

When you send datagrams to the server, using :gen_udp.send/4, you set the destination port to 123. You don’t need special permissions for that.

:gen_udp.open/1/2 opens the port to send and receive. If you just want to send datagrams without receiving, then using 0 as port is usually sufficient. Also remember, that open takes your local port, while the remote port is an argument to send/4. Both ports may be (and in fact almost ever are) different.

I do not know though, how pythons UDP implementation works.