How to add a required value for optional parameter when using argparse in Python3.x

Question

So I am trying to implement argparse in my Python code.
I want to have lets say 4 possible arguments that could be passed. But, if I choose the fourth then I would need an additional argument, but I can't implement this.

parser = argparse.ArgumentParser(description='Testing arguments')
parser.add_argument("--a", action="store_true")
parser.add_argument("--b", action="store_true")
parser.add_argument("--c", action="store_true")
parser.add_argument("--d", action="store_true")
args = parser.parse_args()

Example:

python3 mycode.py --a #no further argument needed  
python3 mycode.py --d further_argument # further argument needed

I have read some of the posts on Stackoverflow, but I couldn't really find the thing I need.

Can you use '--d' as the default 'store' Action?

hpaulj
– hpaulj

2019-06-27 16:35:17 +00:00
Commented Jun 27, 2019 at 16:35 — hpaulj
– hpaulj, Commented Jun 27, 2019 at 16:35

mx0 · Accepted Answer · 2019-06-29 19:26:26Z

1

To make an optional argument take a value you have to add it like this:

parser.add_argument("--d", nargs=1)

it means that if --d is specified than it needs one (and only one) value. nargs keyword can also have *, ?, + for different type of optional values.
Default value for --d when not specified will be None but this can be changed with default=False.

# --a doesn't need a value
>>> parser.parse_args('--a'.split())
Namespace(a=True, b=False, c=False, d=None)

# --d does need a value
>>> parser.parse_args('--d'.split())
usage: [-h] [--a] [--b] [--c] [--d D]
: error: argument --d: expected 1 argument

>>> parser.parse_args('--d VALUE'.split())
Namespace(a=False, b=False, c=False, d=['VALUE'])

When specified, --d value will always be type of list with a single string (for nargs=1).

Also because you are using -- with single letter arguments it makes them long optional arguments that cannot be stacked together. If you change it to - you can run your program like this:

>>> parser.parse_args('-abcd VALUE'.split())
Namespace(a=True, b=True, c=True, d=['VALUE'])

edited Jun 29, 2019 at 19:26

answered Jun 29, 2019 at 19:06

mx0

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1 Comment

Tricker Macedonia Over a year ago

full answer, well written. I think, I will improve my code with your suggestions. I have already solved my problem by just removing the "action=..." part, but again, your answer will be selected as the answer, since you provided additional information.

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