import socket
import multiprocessing

def socketpair():
    l = socket.socket()
    l.bind(('localhost', 0))
    l.listen(1)
    s = socket.socket()
    s.connect(l.getsockname())
    a, _ = l.accept()
    return s, a

def bar(*args):
    s = socket.fromfd(*args)
    print(s)
    s.sendall(b'from bar')

if __name__ == '__main__':
    a, b = socketpair()
    args = (b.fileno(), b.family, b.type, b.proto)
    p = multiprocessing.Process(target=bar, args=args)
    p.start()
    b.close()
    print(a.recv(100))