optional nargs variable in argparse

Question

How I can do conditional loop with argparse with variable nargs? So, basically, it should run with or without argument. I am trying:

parser = argparse.ArgumentParser(description="output parser")
group = parser.add_mutually_exclusive_group()
group.add_argument("--dos", help="get DOSCAR for plot",
                   nargs="?", metavar=("int"))
args = parser.parse_args()

if args.dos:    
    if len(args.dos) > 1:
        chosen = int(args.dos[0])
        chdos = "at_dos"+args.dos[0]+".dat"
    else:
        chosen = None

    inpt = "DOSY"
    print(chosen)
    print(inpt)

Now, if I have variable, then its printing some value, wrong but some value:

$python3 vasp.py --dos 111
111
None   # IT SHOULDN'T BE NONE
DOSY

but nothing without argument.

I have also tried with the normal sys.argv, as:

def get_dos():
    if len(sys.argv) > 2:
        chosen = int(sys.argv[2])
        chdos = "at_dos"+sys.argv[2]+".dat"
    else:
        chosen = None
    inpt = "DOSCAR"
    print(sys.argv)

    print(args.dos)
    print(chosen)
    print(inpt)

in this case, when option is present, its giving correct result:

python3 vasp.py --dos 12
['vasp.py', '--dos', '12']
12
12
DOSCAR

but again, nothing without option:

$python3 vasp.py --dos

I have tried with hpaulj's suggestion. It gives:

$python3 tt.py --dos 12
Namespace(dos='12')
1
DOSY

and without argument, its still not printing anything.

I don't get that None when I run it and fix the invalid syntax. What version of Python are you using? I've tried both 3.5.1 and 2.7.11 and it works in both versions. You need a minimal reproducible example. — Morgan Thrapp
– Morgan Thrapp, Commented Jun 29, 2016 at 20:40
Also, I don't think len(args.dos) > 1 does what you think it does. It checks if you've entered a number with more than 1 digit. — Morgan Thrapp
– Morgan Thrapp, Commented Jun 29, 2016 at 20:43
You now need a test that can handle the string '12', not a list or number. A string. — hpaulj
– hpaulj, Commented Jun 29, 2016 at 21:16
What is is supposed to do with just --dos and without it at all? — hpaulj
– hpaulj, Commented Jun 29, 2016 at 21:22

hpaulj · Accepted Answer · 2016-06-30 00:45:45Z

Simplifying your parser in a Ipython session:

In [1004]: parser=argparse.ArgumentParser()    
In [1005]: parser.add_argument('--dos', nargs='?')

In [1007]: parser.parse_args('--dos 111'.split())
Out[1007]: Namespace(dos='111')

In this case args.dos will be the string '111', with len 3, and int(args.dos[0]) the number 1. The same happens in you leave nargs blank (default None).

With nargs='?' I can also use the flag without an argument, in which case the value is the default None.

In [1013]: parser.parse_args('--dos'.split())
Out[1013]: Namespace(dos=None)

nargs=?is most useful with aconst, which gives a convenient 3 way action. I can addtype=int` to convert the string, if any to integer.

In [1015]: parser.add_argument('--dos', nargs='?', type=int,
   default=None, const=123)

In [1016]: parser.parse_args([]) # not used
Out[1016]: Namespace(dos=None)

In [1017]: parser.parse_args('--dos'.split())  # without argument
Out[1017]: Namespace(dos=123)

In [1018]: parser.parse_args('--dos 456'.split())  # with argument
Out[1018]: Namespace(dos=456)

Other nargs, like 1, '*' and '+' give you a list, which you can check for length etc.

====================

In your argv testing

if len(sys.argv) > 2:
    chosen = int(sys.argv[2])
    chdos = "at_dos"+sys.argv[2]+".dat"

sys.argv is a list, so the len counts if there are enough elements to apply the sys.argv[2] step.

This does not work because args.dos is a single string, not a list.

if len(args.dos) > 1:
    chosen = int(args.dos[0])
    chdos = "at_dos"+args.dos[0]+".dat"

len(args.dos) is then the number of characters in the string, and args.dos[0] is the first character.

============

If I define:

def get_dos(argv=None):
    parser=argparse.ArgumentParser()
    parser.add_argument('--dos', type=int, nargs='?')
    args = parser.parse_args(argv)
    chosen = args.dos
    if chosen is not None:
        chdos = 'at_dos%s.dat'%chosen
    else:
        chdos = ''
    return chosen, chdos

these tests produce values which I think match your needs:

In [1042]: get_dos([])
Out[1042]: (None, '')

In [1043]: get_dos(['--dos'])
Out[1043]: (None, '')

In [1044]: get_dos(['--dos','123'])
Out[1044]: (123, 'at_dos123.dat')

bpachev · Accepted Answer · 2016-06-29 20:47:37Z

0

You need to replace nargs = "?" with nargs = "*". Setting nargs to "?" means there will be 1 optional argument. What you want is a list of optional arguments, which is "*". Like so.

import argparse
parser = argparse.ArgumentParser(description="output parser")
group = parser.add_mutually_exclusive_group()
group.add_argument("--dos", help="get DOSCAR for plot",
               nargs="*", metavar=("int"))
args = parser.parse_args()

if args.dos:    
    if len(args.dos) > 1:
        chosen = int(args.dos[0])
        chdos = "at_dos"+args.dos[0]+".dat"
    else:
        chosen = None

    inpt = "DOSY"
    print(chosen)
    print(inpt)

answered Jun 29, 2016 at 20:47

bpachev

2,22217 silver badges17 bronze badges

1 Comment

BaRud Over a year ago

I have already tried nargs = "*". Have you checked the code? its giving same problem.

BaRud · Accepted Answer · 2016-06-30 10:35:36Z

thanks to all the people who replied, and specially @hpaulj. But none of them really solve my problem. So, I took other way. I am posting this here for completeness.

#!/usr/bin/python3
import argparse

parser = argparse.ArgumentParser(description="output parser")
group = parser.add_mutually_exclusive_group()
group.add_argument("--dos", help="get DOSCAR for plot",action='store_true')
parser.add_argument("-n", help="Showing last n line",
                    metavar='integer', type=int)

args = parser.parse_args()
if args.dos:
    if args.n:
        chosen = int(args.n)
        chdos = "at_dos"+str(args.n)+".dat"
    else:
        chosen = None
    inpt = "DOSY"
    print(inpt)
    print(chosen)

which yields expected results properly:

$python3 tt.py --dos
DOSY
None

$python3 tt.py --dos -n 222
DOSY
222

Collectives™ on Stack Overflow

optional nargs variable in argparse

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related