#! /usr/bin/env python
#filename:pwd.py
# development environment:python2.51


import getpass
usr=getpass.getuser()


while True:
    pwd=getpass.getpass("passwd:%s:" % usr)
    if pwd=='123':
        print "welcome to python!!!!"
        break
    else:
        print "The password you entered is incorrect"

I'm not sure it work on mac
getpass.getuser() Return the “login name” of the user. Availability: Unix, Windows.