changing the behaviour of argument having default value

Question

I have a script that has a argument which has a default value. I want to do something:

$sample.py => Dont do anything
$sample.py -R => Use default value of -R
$sample.py -R <value> =>do something based on value passed

Is this doable in argparse?

Hi all. Please note that my problem is different from the one mentioned in the solution.

Basically i want action corresponding to -R to be performed only when i pass -R on command line. If only -R is passed then value of -R is default value.

If i run the py file without -R then no action corresponding to -R should be done.

Possible duplicate of [Argparse optional positional arguments?](http://stackoverflow.com/questions/4480075/argparse-optional-positional-arguments) — Mast, Dec 16 '15 at 12:50
This looks like an optional optional argument, not a positional. :) — hpaulj, Dec 16 '15 at 21:30

score 3 · Answer 1 · answered Dec 16 '15 at 21:32

3

parser.add_argument('-R',nargs='?', default='default',const='const')

should produce

test.py => namspace(R='default')
test.py -R => namespace(R='const')
test.py -R value => namespace(R='value')

This is documented in https://docs.python.org/3/library/argparse.html#nargs, subsection nargs='?'.

answered Dec 16 '15 at 21:32

hpaulj

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Thanks this solved my issue, took me a while to adjust as per code :) – Amit Goel Dec 17 '15 at 06:55

changing the behaviour of argument having default value

1 Answers1