How to use argparse with both python and program flags?

Question

python -i prog.py -h

When calling the above command I get the expected output with a traceback and 'SystemExit: 0'. Without '-i' I get the expected output without a traceback. Why does this happen and is there a way to use both python flags and program flags in the same command without the traceback?

Difficult to tell without seeing your code. A basic example that I just tried works without issue. What platform are you running on? Windows? Linux? — scotty3785
– scotty3785, Commented Nov 8, 2016 at 17:24

user2357112 · Accepted Answer · 2016-11-08 17:28:29Z

1

Running Python with the -i flag changes the handling of the SystemExit exception, which is used for things like sys.exit. Normally, an uncaught SystemExit causes Python to silently exit. However, with -i on, SystemExit is treated like any other exception, with a traceback and everything.

If you want to silence SystemExit tracebacks with -i on, you'll need to explicitly catch and ignore them. For example,

def main():
    try:
        ...
    except SystemExit:
        # Catch the exception to silence the traceback when running under python -i
        pass

if __name__ == '__main__':
    main()

answered Nov 8, 2016 at 17:28

user2357112

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2 Comments

Jonathan Over a year ago

Ah ok, so this has nothing to do with argparse and everything to do with exiting in interactive mode. Should I go ahead and delete this question to avoid confusion?

user2357112 Over a year ago

@Jay: It seems like the question could be useful to future readers. I'd leave it.

Jared N · Accepted Answer · 2016-11-08 17:31:04Z

1

From the docs:

exception SystemExit
This exception is raised by the sys.exit() function. When it is not handled, the Python interpreter exits; no stack traceback is printed.

argparse employs the SystemExit exception with the '-h' option, and since you enter interactive mode with the command line argument '-i' you see the traceback. Notice that the traceback is not printed if you implement and send in a different option:

python -i prog.py -p 80

Two immediate "quick-fixes" I can think of (but really, it comes down to what do you really need this for?)

Put in a try-except clause when parsing your arguments.
python -i prog.py -h 2> /dev/null

answered Nov 8, 2016 at 17:31

Jared N

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