三次登陆

基础版

    n = 3
    while n > 0:
        username = input('请输入用户名')
        pwd = input('请输入密码')
        if username == 'xiaoqiang' and pwd == '123':
            print('登陆成功')
            break
        else:
            print('登陆失败')
        n -= 1
        print('输入错误，你还有%s机会' % n)

函数版

def login():
    n = 3
    while n > 0:
        username = input('请输入用户名')
        pwd = input('请输入密码')
        if username == 'xiaoqiang' and pwd == '123':
            print('登陆成功')
            break
        else:
            print('登陆失败')
        n -= 1
        print('输入错误，你还有%s机会' % n)