user2125827 user2125827 - 4 months ago 63
Python Question

argparse option for passing a list as option

I am trying to pass a list as an argument to a command line program. Is there an

option to pass a list as option?

parser.add_argument('-l', '--list',
type=list, action='store',
dest='list',
help='<Required> Set flag',
required=True)


Script is called like below

python test.py -l "265340 268738 270774 270817"

Answer

Use the nargs option.

parser.add_argument('-l','--list', nargs='+', help='<Required> Set flag', required=True)

nargs='+' takes 1 or more arguments, nargs='*' takes zero or more.

Let's take a look in more detail at some of the different ways one might try to do this, and the end result.

import argparse

parser = argparse.ArgumentParser()

# By default it will fail with multiple arguments.
parser.add_argument('--default')

# Telling the type to be a list will also fail for multiple arguments,
# but give incorrect results for a single argument.
parser.add_argument('--list-type', type=list)

# This will allow you to provide multiple arguments, but you will get
# a list of lists which is not desired.
parser.add_argument('--list-type-nargs', type=list, nargs='+')

# This is the correct way to handle accepting multiple arguments.
# '+' == 1 or more.
# '*' == 0 or more.
# '?' == 0 or 1.
# An int is the number of arguments to accept.
parser.add_argument('--nargs', nargs='+')

# To make the input integers
parser.add_argument('--nargs-int-type', nargs='+', type=int)

# An alternate way to accept multiple inputs, but you must
# provide the flag once per input. Of course, you can use
# type=int here if you want.
parser.add_argument('--append-action', action='append')

# To show the results of the given option to screen.
for _, value in parser.parse_args()._get_kwargs():
    if value is not None:
        print(value)

Here is the output you can expect:

$ python arg.py --default 1234 2345 3456 4567
...
arg.py: error: unrecognized arguments: 2345 3456 4567
$ python arg.py --list-type 1234 2345 3456 4567
...
arg.py: error: unrecognized arguments: 2345 3456 4567
$ # Quotes won't help here... 
$ python arg.py --list-type "1234 2345 3456 4567"
['1', '2', '3', '4', ' ', '2', '3', '4', '5', ' ', '3', '4', '5', '6', ' ', '4', '5', '6', '7']
$ python arg.py --list-type-nargs 1234 2345 3456 4567
[['1', '2', '3', '4'], ['2', '3', '4', '5'], ['3', '4', '5', '6'], ['4', '5', '6', '7']]
$ python arg.py --nargs 1234 2345 3456 4567
['1234', '2345', '3456', '4567']
$ python arg.py --nargs-int-type 1234 2345 3456 4567
[1234, 2345, 3456, 4567]
$ python arg.py --append-action 1234 --append-action 2345 --append-action 3456 --append-action 4567
['1234', '2345', '3456', '4567']

Takeaways:

  • Use nargs or action='append'
  • Don't use quotes on the command line
  • Don't use type=list, as it will return a list of lists
    • But you can use type=int (or whatever) to get a list of ints (or whatever)
Comments