1 _user="cindy" 2 _pwd="password" 3 passed_authentication = False 4 for i in range (3): 5 username = input("Username=") 6 password = input("Password=") 7  8 if username == _user and password == _pwd: 9 print("Welcome %s login ..." %_user)10 # passed_authentication = True11 break12 else:13 print("Invaild username or password! please retry")14 else: print("Do not try login again!") #for后面加else 只要for执行完整的循环 不中断循环 就会执行else里面的语句15 # if not passed_authentication == True:16 # print ("Do not try login again!")17 # for i in range(1,101,3):18 #19 # print ("loop=",i)