捕获异常时sys.exit（）不会退出程序

Question

This program checks molecular formulas. I want the program to exit as soon as it detects an error in a formula. For example, the formula "a", is incorrect. When I run it through my code:

def readletter():
    if q.peek() in string.ascii_lowercase:
        print(q.peek())
        return q.get()
    else:
        raise Formelfel("Förväntad liten bokstav.")

def readLetter():
    if q.peek() in string.ascii_uppercase:
        print(q.peek())
        return q.get()
    else:
        raise Formelfel("Förväntad stor bokstav.")

def readAtom():
    X = ""
    try:
        X += readLetter()
    except Formelfel:
        print("Missing capital letter at end of row "+getRest())
        sys.exit()
        return

    try:
        x = readletter()
        atom = X+x
    except (Formelfel, TypeError):
        atom = X

    if atom in ATOMER:
        return
    else:
        raise Formelfel("Okänd atom.")

def readGroup():
    if q.peek() in string.ascii_uppercase or q.peek() in string.ascii_lowercase:
        try:
            readAtom()
        except:
            print("Unknown atom at end of row "+getRest())
            sys.exit()

I get this output:

Missing capital letter and end of row a
Unknown atom at end of row

Why is this? I called sys.exit() before print("Unknown atom at end of row "+getRest()) so why does it still execute? I want only the first row of the output to be printed.

Answer 1

sys.exit raises a SystemExit exception. You are catching it with your except clause.

What you should do instead is catch a more specific class of exceptions, which does not include SystemExit .

Catching Exception will work:

def readGroup():
    if q.peek() in string.ascii_uppercase or q.peek() in string.ascii_lowercase:
        try:
            readAtom()
        except Exception:
            print("Unknown atom at end of row "+getRest())
            sys.exit()

You can learn more about exceptions and SystemExit in the docs .

---

Note that you should ideally catch something more specific than Exception (which is very broad, and may catch exceptions you don't intend to catch).

Answer 2

因为在python中，退出事件被作为SystemExit异常处理