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"
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Answer
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.
Don’t use type=list
!!! – There is probably no situation where you would want to use type=list
with argparse
. Ever.
LONG ANSWER
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
oraction='append'
nargs
can be more straightforward from a user perspective, but it can be unintuitive if there are positional arguments becauseargparse
can’t tell what should be a positional argument and what belongs to thenargs
; if you have positional arguments thenaction='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 as4
) then there will be no problem mixing options withnargs
and positional arguments becauseargparse
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 oftype
to coerce each individual given argument you your chosentype
, not the aggregate of all arguments. - You can use
type=int
(or whatever) to get a list of ints (or whatever)
- This happens because under the hood
1: I don’t mean in general.. I mean using quotes to pass a list to argparse
is not what you want.