sys.exit(n)和exit()、os._exit(n)

1. os._exit(n)

直接退出, 不拋異常, 不執行相關清理工作. 常用在子進程的退出.

Note that there is a third exit option, namely os._exit, which exits without calling cleanup handlers, flushing stdio buffers, etc. (and which should normally only be used in the child process after a fork()).

2. sys.exit(n) 

退出程序引發SystemExit異常, 可以捕獲異常執行些清理工作. n默認值爲0, 表示正常退出. 其他都是非正常退出. 還可以sys.exit("sorry, goodbye!"); 一般主程序中使用此退出.

3. exit()/quit()

拋出SystemExit異常. 一般在交互式shell中退出時使用.

---

sys.exit()和exit()， they do mostly the same: raising SystemExit. 

sys.exit does so in sysmodule.c:

static PyObject *
sys_exit(PyObject *self, PyObject *args)
{
    PyObject *exit_code = 0;
    if (!PyArg_UnpackTuple(args, "exit", 0, 1, &exit_code))
        return NULL;
    /* Raise SystemExit so callers may catch it or clean up. */
    PyErr_SetObject(PyExc_SystemExit, exit_code);
   return NULL;
}

While exit is defined in site.py:

class Quitter(object):
    def __init__(self, name):
        self.name = name
    def __repr__(self):
        return 'Use %s() or %s to exit' % (self.name, eof)
    def __call__(self, code=None):
        # Shells like IDLE catch the SystemExit, but listen when their
        # stdin wrapper is closed.
        try:
            sys.stdin.close()
        except:
            pass
        raise SystemExit(code)
__builtin__.quit = Quitter('quit')
__builtin__.exit = Quitter('exit')

---

The site module (which is imported automatically during startup, except if the -S command-line option is given) adds several constants to the built-in namespace (e.g. exit). They are useful for the interactive interpreter shell and should not be used in programs.