先注册：

import hashlib, json
f=open('passwordbank.txt','w')
db={}

def get_md5(password):
    md5=hashlib.md5()
    md5.update(password.encode('utf-8'))
    return md5.hexdigest()

def register(user,password):
    if user in db.keys():
        print('该用户名已存在！请重新输入！')
    else:
        db[user] = get_md5(password + user + 'xinhui')

json.dump(db,f)
f.close()

再登录：

f=open('passwordbank.txt','r')
db=json.load(f)

def login(user,password):
    md5=hashlib.md5()
    md5.update(password + user + 'xinhui',encode('utf-8'))
    passWord=md5.hexdigest()
    if db[user] == passWord:
        print('登录成功！')
    else:
        print('密码错误！')

f.close()