#!/usr/bin/env python

db = {}

def newuser():
  prompt = 'login desired: '
  while True:
    name = raw_input(prompt)
    if db.has_key(name):
      prompt = 'name taken, try another\n'
      continue
    else:
      break

  pwd = raw_input('passwd: ')
  db[name] = pwd

def olduser():
  name = raw_input('login: ')
  pwd = raw_input('passwd: ')

  passwd = db.get(name)
  if passwd == pwd:
    print 'welcome back', name
  else:
    print 'login incorrect'

def showmenu():
  prompt = """

(N)ew User Login
(E)xisting User Login
(Q)uit

Enter choice:"""
  done = False
  while not done:

    chosen = False
    while not chosen:
      try:
        choice = raw_input(prompt).strip()[0].lower()
      except:
        choice = 'q'
      print '\nYou picked: [%s]' % choice
      if choice not in 'neq':
        print 'invalid option, try again'
      else:
        chosen = True

    if choice == 'q':done = True
    if choice == 'n':newuser()
    if choice == 'e':olduser()

if __name__ == '__main__':
  showmenu()

[root@localhost src]# python usrpw.py 


(N)ew User Login
(E)xisting User Login
(Q)uit

Enter choice:n

You picked: [n]
login desired: root
passwd: 1


(N)ew User Login
(E)xisting User Login
(Q)uit

Enter choice:n

You picked: [n]
login desired: root
name taken, try another