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