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 argparse 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"

TL;DR

Use the nargs option or the 'append' setting of the action option (depending on how you want the user interface to behave).

nargs 

parser.add_argument('-l','--list', nargs='+', help='<Required> Set flag', required=True)
# Use like:
# python arg.py -l 1234 2345 3456 4567

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

append 

parser.add_argument('-l','--list', action='append', help='<Required> Set flag', required=True)
# Use like:
# python arg.py -l 1234 -l 2345 -l 3456 -l 4567

With append you provide the option multiple times to build up the list.

Don't use type=list !!! - There is probably no situation where you would want to use type=list with argparse . Ever.

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 an explicit 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]

$ # Negative numbers are handled perfectly fine out of the box.
$ 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'
  • nargs can be more straightforward from a user perspective, but it can be unintuitive if there are positional arguments because argparse can't tell what should be a positional argument and what belongs to the nargs ; if you have positional arguments then action='append' may end up being a better choice.
  • The above is only true if nargs is given '*' , '+' , or '?' . If you provide an integer number (such as 4 ) then there will be no problem mixing options with nargs and positional arguments because argparse will know exactly how many values to expect for the option.
  • Don't use quotes on the command line1
  • Don't use type=list , as it will return a list of lists
  • This happens because under the hood argparse uses the value of type to coerce each individual given argument you your chosen type , not the aggregate of all arguments.
  • You can use type=int (or whatever) to get a list of ints (or whatever)

    • 1: I don't mean in general.. I mean using quotes to pass a list to argparse is not what you want.

    I prefer passing a delimited string which I parse later in the script. The reasons for this are; the list can be of any type int or str , and sometimes using nargs I run into problems if there are multiple optional arguments and positional arguments. 

    parser = ArgumentParser()
parser.add_argument('-l', '--list', help='delimited list input', type=str)
args = parser.parse_args()
my_list = [int(item) for item in args.list.split(',')]

    Then, 

    python test.py -l "265340,268738,270774,270817" [other arguments]

    or, 

    python test.py -l 265340,268738,270774,270817 [other arguments]

    will work fine. The delimiter can be a space, too, which would though enforce quotes around the argument value like in the example in the question.

    除了nargs ，如果您事先知道该列表，您可能需要使用choices ： 

    >>> parser = argparse.ArgumentParser(prog='game.py')
>>> parser.add_argument('move', choices=['rock', 'paper', 'scissors'])
>>> parser.parse_args(['rock'])
Namespace(move='rock')
>>> parser.parse_args(['fire'])
usage: game.py [-h] {rock,paper,scissors}
game.py: error: argument move: invalid choice: 'fire' (choose from 'rock',
'paper', 'scissors')
    链接地址: http://www.djcxy.com/p/61968.html

    上一篇: 作为选项处理argparse转义字符

    下一篇: 作为选项传递列表的argparse选项