Python中，如何从Try中正确跳出一个While循环？

Question

我正在编写一个 Python 函数，使用用户提交的凭据向 URL 发送 GET 请求，并获得一个 returned 令牌供以后使用。

# Function for logging in and get a valid token
def getToken():
    while True: # Loop the cycle of logging in until valid token is received
        try:
            varUsername = raw_input("Enter your username: ")
            varPassword = getpass.getpass("Enter your password: ")
            reqAuthLogin = 'https://MY_URL?username=' + varUsername + '&password=' + varPassword # Send the login request
            varToken =  json.loads(urllib2.urlopen(reqAuthLogin).read())['Token'] # Attempt to parse the JSON response and read the Token, if possible
        except: # If credential is invalid and no token returned
            os.system('cls')
            print 'Invalid credentials. Please try again. \n'
        else: # I want this Try to exit the while loop if login is successful
            break

    os.system('cls')
    return varToken # Return the retrieved token at the end of this function

当username/password组合不正确时，代码抛出KeyError异常。我了解到 Try-Else 可以打破最内层的循环，在这种情况下应该是 while 循环。在我的设计中，我希望函数在发生异常时输出以下消息（意味着无效凭据）：

Invalid credentials. Please try again.

Enter your username:

如果登录成功，代码应该清除屏幕并return获取令牌。问题是，这些代码在不在 Try 而不是在 While 时运行良好。现在它在登录成功时输出：

Enter your username:

显然即使没有异常发生，程序也不会执行Else 分支。我是Python的新手，请帮我找出这个错误的原因。

编辑：感谢评论中的建议。我设置了几个断点，但断点显示即使我在try块的末尾插入一个break，程序也会先执行它然后直接返回到"while True"语句。好像break没有成功退出循环。

Answer 1

为什么不直接添加一个在成功尝试结束时更改的布尔值？

# Function for logging in and get a valid token
def getToken():
    gotToken = False
    while not gotToken: # Loop the cycle of logging in until valid token is received
        try:
            varUsername = raw_input("Enter your username: ")
            varPassword = getpass.getpass("Enter your password: ")
            reqAuthLogin = 'https://MY_URL?username=' + varUsername + '&password=' + varPassword # Send the login request
            varToken =  json.loads(urllib2.urlopen(reqAuthLogin).read())['Token'] # Attempt to parse the JSON response and read the Token, if possible
            gotToken = True
        except: # If credential is invalid and no token returned
            os.system('cls')
            print 'Invalid credentials. Please try again. \n'
    os.system('cls')
    return varToken # Return the retrieved token at the end of this function

Python中，如何从Try中正确跳出一个While循环？

In Python, how to correctly jump out of a While loop from Try?

python

try-catch

try-except