while，for，if输入账号密码判断

AccountNumber1 = []
password1 = []
flag = True
while flag:
    num = 0
    a = 1
    print('-----------这是个欢迎界面---------------')
    print('''
    1、注册账号
    2、登陆账号
    3、更改密码
    ''')
    num=input()

    if num==1:
        while a<2:
            AccountNumber=input('请您注册账号')
            if AccountNumber1.count(AccountNumber)==0:
                AccountNumber1.append(AccountNumber)
                print('您的账号是{}'.format(AccountNumber))
            else:
                print('{}这个账号已经被注册过咯！请进行其他操作！'.format(AccountNumber))
                break
            password=input('请注册您的密码')

            if password1.count(password)==0:
                # print(password1.count(password1))
                password1.append(password)
                print('您的密码是{}'.format(password))
                a+=1
    if num==2:
        x=-1
        EnterAccount=input('请输入您的账号')
        Enterpassword=input ('请输入您的密码')
        for i in AccountNumber1:
            x+=1
            if i==EnterAccount:
                if password1[x]==Enterpassword:
                    print('登陆成功！')
                    flag = False
                    break
                else:
                    print("密码不正确")
            else:
                print('账号不存在')
        else:
            print('账号不存在')

    if num == 3:
        x = -1

        for i in AccountNumber1:
            x += 1
            AccountNumber = input('请输入您的账号')
            OldPassword = input('请输入您的旧密码')
            if i == AccountNumber:
                if password1[x] == OldPassword:
                    NewPassword=input('输入您的新密码')
                    password1[x]=NewPassword
                    print('修改完毕！')
                    flag = False
                    break
                else:
                    print("密码错误，请重新输入旧密码")

            else:
                print('账号不存在或错误')
        else:
            print('账号不存在')