>>> dict1 = {'foo':789, 'foo': 'xyz'} 
  >>> dict1 
  {'foo': 'xyz'} 
  >>> dict1['foo'] = 123 
  >>> dict1 
  {'foo': 123} 

db = {}

def newuser():

  prompt= 'please regist your name: '

  while True:

    name = raw_input(prompt)

    if db.has_key(name):

      prompt = 'name taken,try another: '

      continue

    else:

      break

  pwd = raw_input('passswd: ')

  db[name] = pwd

  print 'Newuser [%s] has added successfully!' %name

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: """

  while True:

    try:

      choice = raw_input(prompt).strip()[0].lower()

      print '\nYou picked: [%s]' % choice

      if choice not in 'neq':

        print 'invalid option,please try again'

      if choice == 'n':

        newuser()

      if choice == 'e':

        olduser()

      if choice == 'q':

        break

    except(EOFError,KeyboardInterrupt):

      print 'invalid option,please try again'


if __name__ == '__main__':

  showmenu()