python – How can I pass a list as a command-line argument with argparse?

SHORT ANSWER

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.

Dont use type=list!!! – There is probably no situation where you would want to use type=list with argparse. Ever.

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LONG ANSWER

Lets 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 wont 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 cant 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.
• Dont use quotes on the command line¹
• Dont 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)

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¹: I dont 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.

Or you can use a lambda type as suggested in the comments by Chepner:

parser.add_argument(-l, --list, help=delimited list input, 
    type=lambda s: [int(item) for item in s.split(,)])

python – How can I pass a list as a command-line argument with argparse?

Additionally to nargs, you might want to use choices if you know the list in advance:

>>> 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)