Using sys.exit or SystemExit; when to use which?

sys.exit(s) is just shorthand for raise SystemExit(s), as described in the former’s docstring; try help(sys.exit). So, instead of either one of your example programs, you can do

sys.exit('Unable to open %s' % reference)

According to documentation sys.exit(s) effectively does raise SystemExit(s), so it’s pretty much the same thing.

SystemExit is an exception, which basically means that your progam had a behavior such that you want to stop it and raise an error. sys.exit is the function that you can call to exit from your program, possibily giving a return code to the system.

EDIT: they are indeed the same thing, so the only difference is in the logic behind in your program. An exception is some kind of “unwanted” behaviour, whether a call to a function is, from a programmer point of view, more of a “standard” action.

My personal preference is that at the very least SystemExit is raised (or even better – a more meaningful and well documented custom exception) and then caught as close to the “main” function as possible, which can then have a last chance to deem it a valid exit or not. Libraries/deeply embedded functions that have sys.exit is just plain nasty from a design point of view. (Generally, exiting should be “as high up” as possible)

There are 3 exit functions, in addition to raising SystemExit.

The underlying one is os._exit, which requires 1 int argument, and exits immediately with no cleanup. It’s unlikely you’ll ever want to touch this one, but it is there.

sys.exit is defined in sysmodule.c and just runs PyErr_SetObject(PyExc_SystemExit, exit_code);, which is effectively the same as directly raising SystemExit. In fine detail, raising SystemExit is probably faster, since sys.exit requires an LOAD_ATTR and CALL_FUNCTION vs RAISE_VARARGS opcalls. Also, raise SystemExit produces slightly smaller bytecode (4bytes less), (1 byte extra if you use from sys import exit since sys.exit is expected to return None, so includes an extra POP_TOP).

The last exit function is defined in site.py, and aliased to exit or quit in the REPL. It’s actually an instance of the Quitter class (so it can have a custom __repr__, so is probably the slowest running. Also, it closes sys.stdin prior to raising SystemExit, so it’s recommended for use only in the REPL.

As for how SystemExit is handled, it eventually causes the VM to call os._exit, but before that, it does some cleanup. It also runs atexit._run_exitfuncs() which runs any callbacks registered via the atexit module. Calling os._exit directly bypasses the atexit step.

While the difference has been answered by many answers, Cameron Simpson makes an interesting point in https://mail.python.org/pipermail/python-list/2016-April/857869.html:

TL;DR: It’s better to just raise a "normal" exception, and use SystemExit or sys.exit only at the top levels of a script.

I m on python 2.7 and Linux , I have a simple code need suggestion if I
I could replace sys.exit(1) with raise SystemExit .

==Actual code==

def main():    
    try:
       create_logdir()
       create_dataset()
       unittest.main()    
     except Exception as e:
       logging.exception(e)
       sys.exit(EXIT_STATUS_ERROR)

if __name__ == '__main__':    main()

==Changed Code==

def main():    
    try:
       create_logdir()
       create_dataset()
       unittest.main()    
    except Exception as e:
       logging.exception(e)
       raise SystemExit

if __name__ == '__main__':    
    main()

I am against both of these personally. My preferred pattern is like
this:

  def main(argv):
    try:
      ...
    except Exception as e:
      logging.exception(e)
      return 1

  if __name__ == '__main__':
    sys.exit(main(sys.argv))

Notice that main() is back to being a normal function with normal
returns.

Also, most of us would avoid the "except Exception" and just let a top
level except bubble out: that way you get a stack backtrace for
debugging. I agree it prevents logging the exception and makes for
uglier console output, but I think it is a win. And if you do want
to log the exception there is always this:

try:
… except Exception as e:
logging.exception(e)
raise

to recite the exception into the log and still let it bubble out
normally.

The problem with the "except Exception" pattern is that it catches and
hides
every exception, not merely the narrow set of specific exceptions that you understand.

Finally, it is frowned upon to raise a bare Exception class. In
python 3 I believe it is actually forbidden, so it is nonportable
anyway. But even In Python to it is best to supply an Exception
instance, not the class:

raise SystemExit(1)

2. All the functions in try block have exception bubbled out using raise

Example for create_logdir() here is the function definition

def create_logdir():

try:
os.makedirs(LOG_DIR)
except OSError as e:
sys.stderr.write("Failed to create log directory…Exiting !!!")
raise
print "log file: " + corrupt_log
return True

def main():
try:
create_logdir()
except Exception as e:
logging.exception(e)
raise SystemExit

(a) In case if create_logdir() fails we will get the below error ,is
this fine or do I need to improve this code.

Failed to create log directory…Exiting !!!ERROR:root:[Errno 17] File
exists: ‘/var/log/dummy’

Traceback (most recent call last):
File "corrupt_test.py", line 245, in main
create_logdir()
File "corrupt_test.py", line 53, in create_logdir
os.makedirs(LOG_DIR)
File "/usr/local/lib/python2.7/os.py", line 157, in makedirs
OSError: [Errno 17] File exists: ‘/var/log/dummy’

I prefer the bubble out approach, perhap with a log or warning
messages as you have done, eg:

logging.exception("create_logdir failed: makedirs(%r): %s" %
(LOG_DIR, e)) raise

(Also not that that log message records more context: context is very
useful when debugging problems.)

For very small scripts sys.stderr.write is ok, but in general any of
your functions that turned out to be generally useful might migrate
into a library in order to be reused; consider that stderr is not
always the place for messages; instead reading for the logging module
with error() or wanr() or exception() as appropriate. There is more
scope for configuring where the output goes that way without wiring
it into your inner functions.

3. Can I have just raise , instead of SystemExit or sys.exit(1) . This
   looks wrong to me

def main():

try:
create_logdir()
except Exception as e
logging.exception(e)
raise

This is what I would do, myself.

Think: has the exception been "handled", meaning has the situation
been dealt with because it was expected? If not, let the exception
bubble out so that the user knows that something not understood by
the program has occurred.

Finally, it is generally bad to SystemExit or sys.exit() from inside
anything other than the outermost main() function. And I resist it
even there; the main function, if written well, may often be called
from somewhere else usefully, and that makes it effectively a library
function (it has been reused). Such a function should not
unilaterally abort the program. How rude! Instead, let the exception
bubble out: perhaps the caller of main() expects it and can handle
it. By aborting and not "raise"ing, you have deprived the caller of
the chance to do something appropriate, even though you yourself
(i.e. "main") do not know enough context to handle the exception.

So I am for "raise" myself. And then only because you want to log the
error. If you didn’t want to log the exception you could avoid the
try/except entirely and have simpler code: let the caller worry
about unhandled exceptions!