Python While 循环突破问题

我的问题是关于 while 循环的标志。这有效，但不像我想象的那样。我认为我不明白某些事情，所以如果有人能够解释，那就太好了。

根据我的理解，一旦满足我的条件之一，这应该会打破循环。因此，如果我输入“q”，它应该会中断并停止循环。但发生的情况是它不断循环，然后爆发。因此它会执行最后一个提示并打印异常。

（Python版本为3.8.5）

# Statement that tells the user what we need.
print("Enter two numbers and I will tell you the sum of the numbers.")
# Lets the user know they can press 'q' to exit the program.
print("Press 'q' at anytime to exit.")

keep_going = True

# Loop to make the program keep going until its told to stop.
while keep_going:
    # Prompt for user to input first number and store it in a variable.
    first_number = input("First number: ")
    # Create a break when entering the first number.
    if first_number == 'q':
        keep_going = False
    # Prompt for user to input second number and store it in a variable.
    second_number = input("Second number: ")
    # Create a break when entering the second number.
    if second_number == 'q':
        keep_going = False
    # Exception for non integers being input "ValueError"
    try:
        # Convert input to integers and add them. 
        # storing the answer in a variable.
        answer = int(first_number) + int(second_number)
    except ValueError:
        # Tell the user what they did wrong.
        print("Please enter a number!")
    else:    
        # Print the sum of the numbers
        print(f"\nThe answer is: {answer}")

使用这段代码，它会像我预期的那样立即爆发。

while True:
    first_number = input("First number: ")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    if second_number == 'q':
        break

我只是想了解有什么区别以及它是否应该如何工作。我觉得我错过了一些东西或者误解了一些东西。

的条件while循环仅在循环体的迭代之间进行检查，因此如果您在循环中间更改条件，则当前迭代将在循环终止之前完成。如果你想打破循环立即地，你需要break（无论条件如何都会自动中断循环）或continue（它跳转到下一次迭代，因此如果条件不再为真，将终止循环）。

Using while True: with a break当您想要停止循环时通常比尝试通过设置和取消设置标志来控制循环要简单得多。

FWIW，我可能会将所有内容放入一个函数中，然后用Exception， 像这样：

print("Enter two numbers and I will tell you the sum of the numbers.")
print("Press 'q' at anytime to exit.")


def input_number(prompt: str) -> int:
    """Ask the user to input a number, re-prompting on invalid input.
    Exception: raise EOFError if the user enters 'q'."""
    while True:
        try:
            number = input(f"{prompt} number: ")
            if number == 'q':
                raise EOFError
            return int(number)
        except ValueError:
            print("Please enter a number!")


while True:
    try:
        numbers = (input_number(n) for n in ("First", "Second"))
        print(f"The answer is: {sum(numbers)}")
    except EOFError:
        break

这使得扩展程序以处理两个以上的输入变得更容易；尝试在“第一”和“第二”后面添加“第三”！ :)