要求：
# -*- coding=utf-8 -*-
user = {'alex':'qwer1234','lilei':'asdf1234','han×××':'zxcv1234','lucy':'qwer1234'}
loop = True
while loop:
count = 0
f = open('blacklist.txt', mode='r', encoding='utf-8')
blacklist = f.read()
f.close()
input_user = input('please input your username:').strip()
if not input_user : continue
if input_user in blacklist:
print('sorry,your username is banned')
continue
for i in user:
input_pw = input('please input your password:').strip()
if input_user not in user:
print('sorry，your username is error')
elif user[input_user] == input_pw:
print('wellcome',input_user)
loop = False
break
else:
count += 1
print('sorry，your password is error,you have %s times'%(3-count))
if count > 2:
f = open('blacklist.txt','a',encoding='utf-8')
f.write('%s '%input_user )
print('sorry,your username is banned')
f.close()
break