1 #first give username and password
 2 i=3
 3 username = 'dog'
 4 password = '123'
 5 while i >= 0:
 6 #exit when there is no chance
 7     if i == 0:
 8         print('您的机会已经用完')
 9         break
10 #check out
11     else:
12         name = input('请输入用户名： ')
13         i -= 1
14         if name == username:
15             key = input('请输入密码： ')
16             #enter when name and key are all correct
17             if key == password:
18                 print('''
19 登录成功
20 用户：%s
21 密码：%s
22 欢迎您
23                 '''%(name,key))
24                 break
25             #密码不正确时重新循环
26             else:
27                 print('您的密码不正确,还有%d次机会'%(i))
28         else:
29             if i == 0:
30                 continue
31             else:
32                 goon = input('您的用户名不正确，是否重新输入：Y/N ')
33                 if goon == 'N':
34                     print('再见')
35                     break
36                 else:
37                     print('请重新输入用户名和密码')