Is there a method that tells my program to quit?

问题:

For the "q" (quit) option in my program menu, I have the following code:

elif choice == "q":
    print()

That worked all right until I put it in an infinite loop, which kept printing blank lines. Is there a method that can quit the program? Else, can you think of another solution?

回答1:

One way is to do:

sys.exit(0)

You will have to import sys of course.

Another way is to break out of your infinite loop. For example, you could do this:

while True:
    choice = get_input()
    if choice == "a":
        # do something
    elif choice == "q":
        break

Yet another way is to put your main loop in a function, and use return:

def run():
    while True:
        choice = get_input()
        if choice == "a":
            # do something
        elif choice == "q":
            return

if __name__ == "__main__":
    run()

The only reason you need the run() function when using return is that (unlike some other languages) you can't directly return from the main part of your Python code (the part that's not inside a function).

回答2:

The actual way to end a program, is to call

raise SystemExit

It's what sys.exit does, anyway.

A plain SystemExit, or with None as a single argument, sets the process' exit code to zero. Any non-integer exception value (raise SystemExit("some message")) prints the exception value to sys.stderr and sets the exit code to 1. An integer value sets the process' exit code to the value:

$ python -c "raise SystemExit(4)"; echo $?
4

回答3:

In Python 3 there is an exit() function:

elif choice == "q":
    exit()

回答4:

See sys.exit. That function will quit your program with the given exit status.

回答5:

Please note that the solutions based on sys.exit() or any Exception may not work in a multi-threaded environment.

Since exit() ultimately “only” raises an exception, it will only exit the process when called from the main thread, and the exception is not intercepted. (doc)

This answer from Alex Martelli for more details.