需要有关 while-loop 的解释（用于控制流程而不是 goto）

Question

我看到一些类似的问题得到了使用 while 循环而不是在 if 语句中“返回”的答案，但我仍在努力弄清楚它是如何工作的。从我到目前为止看到的示例来看，似乎您应该将变量设置为 False 并在条件为“真”时一直循环，但我真的不明白，可能是理解了这是完全错误的。我在下面有一些带有注释的示例代码，我希望能够在语句中“返回”，但我不知道如何真正实现它

welcome = "If you choose to accept, you will get a list of options below. Do you accept the terms?"
terms = "" # True/False according to below

username = input("Enter your username: ")

if not username:
    print("You haven't entered a username! " + username)    # Here i would want it to start over if no username is specified
else:
    print("Hello, " + username + "\n" + welcome)

choise_terms = input("(yes/no)")

if choise_terms == "no":
    terms == False
elif choise_terms == "yes":
    terms == True
else:
    print("You have to type either \"yes\", or \"no\", do you accept the terms? " + choise_terms)    # Here i would want it to start over if either "yes" or "no" is specified
    
# Continue the program if terms are accepted, else close the application

所以据我所知，我应该能够以某种方式将我的 if 语句放在 whileloop 中，只要将另一个变量设置为 True，循环就会继续？

Answer 1

基本上有两种选择。第一种是使用 while True 无限循环运行并使用 break 在满足特定条件后立即终止，例如当 valid¹ 输入用户名。这是您的代码片段的一个版本，其中实施了 while 循环和一些额外的小重构。

welcome = "If you choose to accept, you will get a list of options below. Do you accept the terms?"
yes_no = "You have to type either \"yes\", or \"no\", do you accept the terms?"

while True:
    username = input("Enter your username: ")

    if username:
        print("Hello,", username)
        print(welcome)
        break
    else:
        print("You haven't entered a username!")
    
choices = ["yes", "no"]

while True:
    choice_terms = input("(yes/no)")

    if choice_terms in choices:
        terms = choice_terms == "yes" # shorter check
        break
    else:
        print(yes_no)

或者，您可以使用无效值初始化目标变量，例如 username = ''，然后在循环中将用户输入重新分配给此变量，这样它将继续运行ning 直到用户输入有效的内容：

username = '' # an invalid username

while not username: # or: while username == ''
    username = input("Enter your username: ")

¹ 什么是“有效”或什么不是“有效”取决于您的应用程序。在您的情况下，有效的用户名将是一个长度大于零的字符串的用户名。第二步中的有效选择是字符串“yes”或“no”之一。

Answer 2

我写了这段代码应该可以解决你的问题（或者至少我希望它能解决）：

welcome = "If you choose to accept, you will get a list of options below. Do you accept the terms?"
terms = "" # True/False according to below

username = input("Enter your username: ")

while not username:
    print("You haven't entered a username! " + username)    # Here i would want it to start over if no username is specified
    username = input("Enter your username: ")
print("Hello, " + username + "\n" + welcome)

choise_terms = input("(yes/no)")

if choise_terms == "no":
    terms == False
elif choise_terms == "yes":
    terms == True
else:
    print("You have to type either \"yes\", or \"no\", do you accept the terms? " + choise_terms)    # Here i would want it to start over if either "yes" or "no" is specified
    
# Continue the program if terms are accepted, else close the application

如您所见，我将错误消息封装在一个 while 循环中，这样程序就不会运行当用户未输入有效 password.Python 时，每次用户都会检查答案输入密码，只有当它最终拥有有效的用户名时，它才会执行程序。希望我能帮到你，问我是否需要更多信息。

需要有关 while-loop 的解释（用于控制流程而不是 goto）

Need explanation about while-loop (use for control flow instead of goto)

python

goto

while-loop