登录操作（方法一：设置flag标志位）

user_name="star"
passwoed='123'
passed_authentication=False
count=0
for i in range(3):
    u_username=input("please input your username:")
    u_password=input("please input your password:")

    if u_username==user_name and u_password==passwoed:
        print("welcome to %s"%u_username)
        passed_authentication=True
        break
    else:
        count+=1
        print("your username or password is Error")
if not passed_authentication:
    count==3
    print("you have try more times,would you want to try again? please input yes or no,please choose")
    answer=input("your choose:")
    if answer=='yes':
        print("go away,hahahah")

Input:正确操作

please input your username:qwe
please input your password:222
your username or password is Error
please input your username:star
please input your password:123
welcome to star

进程已结束,退出代码0

Input:错误操作

please input your username:aa
please input your password:aa
your username or password is Error
please input your username:aa
please input your password:aa
your username or password is Error
please input your username:aa
please input your password:aa
your username or password is Error
you have try more times,would you want to try again? please input yes or no,please choose
your choose:yes
go away,hahahah

进程已结束,退出代码0