First try

async.py

# methods on_read and on_write was called when select confirmed that socket is ready to read from / write to

STATE_SEND_LOGIN = 1
STATE_RECEIVE_LOGIN = 2
STATE_SEND_PASS = 3
STATE_RECEIVE_PASS = 4
STATE_SEND_OK = 5

class Authenticator:
    def __init__(self, sock):
        self.sock = sock
        self.state = STATE_SEND_LOGIN

    def on_read(self):
        if self.state == ...

    def on_write(self):
        if self.state == ...