Go

package utils
import (
    "net"
)
func GetFreePort() (int, error) {
    addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
    if err != nil {
        return 0, err
    }
    l, err := net.ListenTCP("tcp", addr)
    if err != nil {
        return 0, err
    }
    defer l.Close()
    return l.Addr().(*net.TCPAddr).Port, nil
}

Python

def get_free_tcp_port():
    tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    tcp.bind(("", 0))
    _, port = tcp.getsockname()
    tcp.close()
    return port