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

Question

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   lt Required gt  Set flag                         required True    Script is called like below  python test py -l  265340 268738 270774 270817

User · Answer

You can parse the list as a string and use of the eval builtin function to read it as a list. In this case, you will have to put single quotes into double quote (or the way around) in order to ensure successful string parse.

# declare the list arg as a string
parser.add_argument('-l', '--list', type=str)

# parse
args = parser.parse()

# turn the 'list' string argument into a list object
args.list = eval(args.list)
print(list)
print(type(list))

Testing:

python list_arg.py --list "[1, 2, 3]"

[1, 2, 3]
<class 'list'>

User · Answer

I want to handle passing multiple lists  integer values and strings    Helpful link    How to pass a Bash variable to Python   def main args       my args          for arg in args          if arg startswith      and arg endswith                   arg   arg replace          replace                      my args append arg split               else              my args append arg       print my args    if   name         main         import sys     main sys argv 1        Order is not important  If you want to pass a list just do as in between     and    and seperate them using a comma   Then   python test py my string 3   1 2     3 4 5     Output      my string    3     1    2      3    4    5     my args variable contains the arguments in order

User · Answer

In add argument    type is just a callable object that receives string and returns option value   import ast  def arg as list s                                                                   v   ast literal eval s                                                          if type v  is not list                                                              raise argparse ArgumentTypeError  Argument    s   is not a list     s       return v                                                                      def foo        parser add argument  --list   type arg as list  default                             help  List of values     This will allow to       tool --list   1 2 3 4

User · Answer

Using nargs parameter in argparse s add argument method I use nargs     as an add argument parameter  I specifically used nargs     to the option to pick defaults if I am not passing any explicit arguments Including a code snippet as example  Example  temp args1 py Please Note  The below sample code is written in python3  By changing the print statement format  can run in python2    usr local bin python3 6  from argparse import ArgumentParser  description    testing for passing multiple arguments and to get list of args  parser   ArgumentParser description description  parser add argument  -i    --item   action  store   dest  alist                       type str  nargs      default   item1    item2    item3                        help  quot Examples  -i item1 item2  -i item3 quot   opts   parser parse args    print  quot List of items     quot  format opts alist    Note  I am collecting multiple string arguments that gets stored in the list - opts alist If you want list of integers  change the type parameter on parser add argument to int Execution Result  python3 6 temp agrs1 py -i item5 item6 item7 List of items    item5    item6    item7    python3 6 temp agrs1 py -i item10 List of items    item10    python3 6 temp agrs1 py List of items    item1    item2    item3

User · Answer

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   lt Required gt  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   lt Required gt  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

User · Answer

Additionally to nargs  you might want to use choices if you know the list in advance    gt  gt  gt  parser   argparse ArgumentParser prog  game py    gt  gt  gt  parser add argument  move   choices   rock    paper    scissors     gt  gt  gt  parser parse args   rock    Namespace move  rock    gt  gt  gt  parser parse args   fire    usage  game py  -h   rock paper scissors  game py  error  argument move  invalid choice   fire   choose from  rock    paper    scissors

User · Answer

If you have a nested list where the inner lists have different types and lengths and you would like to preserve the type  e g      1  2     foo    bar     3 14   baz   20    then you can use the solution proposed by  sam-mason to this question  shown below   from argparse import ArgumentParser import json  parser   ArgumentParser   parser add argument  -l   type json loads  parser parse args   -l      1 2    foo   bar    3 14  baz  20        which gives   Namespace l   1  2     foo    bar     3 14   baz   20

User · Answer

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  quot 265340 268738 270774 270817 quot   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

User · Answer

I think the most elegant solution is to pass a lambda function to  type   as mentioned by Chepner  In addition to this  if you do not know beforehand what the delimiter of your list will be  you can also pass multiple delimiters to re split     python3 test py -l  abc xyz  123   import re import argparse  parser   argparse ArgumentParser description  Process a list    parser add argument  -l    --list                       type lambda s  re split         s                       required True                      help  comma or space delimited list of characters    args   parser parse args   print args list      Output    abc    xyz    123

User · Answer

If you are intending to make a single switch take multiple parameters  then you use nargs      If your example  -l  is actually taking integers   a   argparse ArgumentParser   a add argument       -l    --list      either of this switches     nargs              one or more parameters to this switch     type int            parameters  are ints     dest  list         store in  list       default            since we re not specifying required     print a parse args  -l 123 234 345 456  split       print a parse args  -l 123 -l 234 -l345 --list 456  split         Produces  Namespace list  123  234  345  456   Namespace list  456      Attention    If you specify the same argument multiple times  the default action   store   replaces the existing data   The alternative is to use the append action   a   argparse ArgumentParser   a add argument       -l    --list      either of this switches     type int            parameters  are ints     dest  list         store in  list       default            since we re not specifying required      action  append     add to the list instead of replacing it    print a parse args  -l 123 -l 234 -l345 --list 456  split         Which produces  Namespace list  123  234  345  456     Or you can write a custom handler action to parse comma-separated values so that you could do  -l 123 234 345 -l 456

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

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