利用循环语句完善输入设置

使用for循环：

代码1：
_user = "alex"
_password = "abc123"

for i in range(3):
    username = input("Username:")
    password = input("Password:")
    if username == _user and password == _password:
        print("Welcome %s login ..." % _user)
        break
    else:
        print("Invalid username or password !")
代码2：

_user = "alex"
_password = "abc123"

passed_authentication = False

for i in range(3):
    username = input("Username:")
    password = input("Password:")
    if username == _user and password == _password:
        print("Welcome %s login ..." % _user)
        passed_authentication = True #真，成立
        break
    else:
        print("Invalid username or password !")

if not passed_authentication:
    print("您的输入次数已经达到上限！") #只有在True的情况下，条件成立

 代码3：

_user = "alex"
_password = "abc123"

#passed_authentication = False

for i in range(3):
    username = input("Username:")
    password = input("Password:")
    if username == _user and password == _password:
        print("Welcome %s login ..." % _user)
        #passed_authentication = True #真，成立
        break
    else:
        print("Invalid username or password !")
else:
    print("您的输入次数已经达到上限！")#for循环正常结束，就可以执行下面的else语句

使用while循环：

代码：

_user = "alex"
_password = "abc123"

counter = 0
while counter < 3:
    username = input("Username:")
    password = input("Password:")
    if username == _user and password == _password:
        print("Welcome %s login ..." % _user)
        break
    else:
        print("Invalid username or password !")
    counter += 1
else:
    print("您的输入次数已经达到上限！")

 代码2

_user = "alex"
_password = "abc123"

counter = 0
while counter < 3:
    username = input("Username:")
    password = input("Password:")
    if username == _user and password == _password:
        print("Welcome %s login ..." % _user)
        break
    else:
        print("Invalid username or password !")
    counter += 1
    if counter == 3:
        keep_going_choice = input("还想玩么？[y/n]")
        if keep_going_choice == "y":
            counter = 0
else:
    print("您的输入次数已经达到上限！")