sys.exit(n)和exit()、os._exit(n)

1. os._exit(n)

直接退出, 不抛异常, 不执行相关清理工作. 常用在子进程的退出.

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) 

退出程序引发SystemExit异常, 可以捕获异常执行些清理工作. n默认值为0, 表示正常退出. 其他都是非正常退出. 还可以sys.exit("sorry, goodbye!"); 一般主程序中使用此退出.

3. exit()/quit()

抛出SystemExit异常. 一般在交互式shell中退出时使用.

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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')

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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.