Python: using sys.exit or SystemExit differences and suggestions

Question

Reading online some programmers use sys.exit, others use SystemExit.
Sorry for the basic question:

1. What is the difference?
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Answer 1

While the difference has been answered by many answers, https://mail.python.org/pipermail/python-list/2016-April/707470.html makes an interesting point:

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!