[Tutor] exit message

[Zachary Ware]

On Sun, May 5, 2013 at 11:24 PM, Jim Mooney <cybervigilante at gmail.com> wrote:
>
> I've noticed that if you exit() a program you always get a traceback message:
> 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?

I believe that traceback is a bug in IDLE, which I think has been
fixed for the next release (or is being chased, one of the two).  If
you're not using IDLE, it's another bug that should be tracked down
:).  More generally, though, you can't count on 'exit()' being
available.  exit() is made available by the 'site' module (as you can
see in the traceback), but if Python is run with the -S option, 'site'
won't be imported and 'exit()' won't be available.

Instead, you can do what exit() actually does, which is to raise the
SystemExit exception with the code to be returned to the shell.  For
instance (and be warned that I'm typing this from a Linux box, so it's
untested, but you should be able to reproduce this :)):

C:\py> type example.py
# example.py
print('Hello')

raise SystemExit(13)

print('Goodbye')

C:\py> py -3.3 example.py
Hello

C:\py> echo %ERRORLEVEL%
13

You can also import sys and use sys.exit() for the same effect.

If you don't care about the exit code, you can just "drop off" the end
of the script.  If you don't end with an unhandled exception, your
exit code will be 0, otherwise it will (probably) be 1.  For example:

# example2.py

def main(arg=None):
    print('This is a main function.  It's not special, you still have
to call it.')
    if arg:
        return
    raise ValueError('If you had supplied an argument, you wouldn't
get this message')

if __name__ == '__main__':
    import sys
    if len(sys.argv) > 1:
        main(sys.argv[1])
    else:
        main()


Hope this helps,

Zach Ware