In which a socket connects to itself. http://www.johncalsbeek.com/2014/10/19/simultaneous-initiation.html

simultaneous_initiation.py

import socket
create_socket = lambda: socket.socket(socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_TCP)

target_port = 49400  # Any port in the ephemeral port range that no process on your machine is using.
port = 0

# Create and bind sockets, working our way through the ephemeral port space, until we get close to our target.
while not (target_port - 5 < port < target_port):
    sock = create_socket()
    sock.bind(('127.0.0.1', 0))
    host, port = sock.getsockname()
    print("Skipping port {}...".format(port))

# For the last few ports, try to connect to the target port.
# On Mac OS X 10.10 (14A389), I get: Unable to connect to port 49400, used local port 49400: [Errno 22] Invalid argument
# On Windows 8.1 (6.3.9600.17328), I get: Connected to port 49400, used local port 49400
while port < target_port:
    sock = create_socket()
    try:
        sock.connect(('127.0.0.1', target_port))
        host, port = sock.getsockname()
        print("Connected to port {}, used local port {}".format(target_port, port))
    except socket.error as e:
        host, port = sock.getsockname()
        print("Unable to connect to port {}, used local port {}: {}".format(target_port, port, e))