[Tutor] exit message

[Steven D'Aprano]

On Sun, May 05, 2013 at 09:24:30PM -0700, Jim Mooney wrote:
> I've noticed that if you exit() a program you always get a traceback message:

Which exit are you talking about? There is sys.exit, and there is also 
the interactive exit() function defined by the site module for 
interactive use.



> Traceback (most recent call last):
>   File "<pyshell#1>", line 1, in <module>
>     exit('what now?')
>   File "C:\Python33\lib\site.py", line 380, in __call__
>     raise SystemExit(code)
> 
> What if you just want to exit for some normal reason and don't want
> the message? Or is a program always supposed to end in some normal way
> without an exit. Or is there a different, more graceful way to end a
> program when you want?

The normal way to exit a program is to just "fall out the bottom" when 
you reach the end. Once all loops are ended, and you reach the end of 
code, Python just neatly exits with no error.

Another alternative is to call sys.exit(), either with no argument, or 
an argument of 0. The convention for command-line tools is that an exit 
value of 0 means "no problem, the application ended normally". If you 
pass any other value, Python will generate a non-zero exit code for you.

A third way is to call down fire from the heavens and bring about the 
apocolypse for your program by calling os._exit(), which literally kills 
the application dead without giving it a chance to clean up after 
itself, close files, etc. Don't use this unless you know what you're 
doing. It also takes a status code argument like sys.exit.

Last but not least, if you raise SystemExit (or for that matter any 
exception) which is not caught by a try...except block, Python will exit 
your program with a non-zero exit code.

As always, IDLE may play silly tricks. You should not run programs "for 
real" under IDLE, but only while you are actively writing code, testing, 
etc. That's why IDLE is called an Interactive Development Environment -- 
if you're not interactively developing, don't use it.

(Actually, I am a great believer in not using IDLE for anything. If your 
application acts weird, is that because it has a bug, or because IDLE is 
doing something "clever"? I much prefer a text editor and a terminal 
window. But then I'm spoilt because I use Linux, where the standard 
environment is practically an IDE on its own.)


-- 
Steven