db={}

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

pwd=raw_input('password:')
db[name]=pwd
           
def olduser():
    name=raw_input('login:')
    pwd=raw_input('passwd:')
    password=db.get(name)
    if passwo==pwd:
        print 'welcome back',name
    else:
        print 'login incorrect'

def showmenu():
    prompt="""
(N)ew User Login
(E)xisting User login
(Q)uit

 qinaideyonghu"""

done=False
while not chosen:
     try:
          choice=raw_input(prompt).strip()[0].lower()
     except (EOFError,KeyboardInterrupt):
          choice='q'
     print '\nbaoqian:[%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()

打开时 可以。username:    然后输出任何数值后显示db[name]=pwd     name没有被定义  我看不出来