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 = True
11         break
12     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)