function_that_needs_strings(*my_list) # works!

You can read all about it here: Unpacking Argument Lists - The Python Tutorial

When you define your function using this syntax:

def someFunc(*args):
    for x in args
        print x

You're telling it that you expect a variable number of arguments. If you want to pass in a List (Array from other languages) you'd do something like this:

def someFunc(myList = [], *args):
    for x in myList:
        print x

Then you can call it with this:

items = [1,2,3,4,5]

someFunc(items)

You need to define named arguments before variable arguments, and variable arguments before keyword arguments. You can also have this:

def someFunc(arg1, arg2, arg3, *args, **kwargs):
    for x in args
        print x

Which requires at least three arguments, and supports variable numbers of other arguments and keyword arguments.

You can use the * operator before an iterable to expand it within the function call. For example:

timeseries_list = [timeseries1 timeseries2 ...]
r = scikits.timeseries.lib.reportlib.Report(*timeseries_list)

(notice the * before timeseries_list)

From the python documentation:

If the syntax *expression appears in the function call, expression must evaluate to an iterable. Elements from this iterable are treated as if they were additional positional arguments; if there are positional arguments x1, ..., xN, and expression evaluates to a sequence y1, ..., yM, this is equivalent to a call with M+N positional arguments x1, ..., xN, y1, ..., yM.

This is also covered in the python tutorial, in a section titled Unpacking argument lists, where it also shows how to do a similar thing with dictionaries for keyword arguments with the ** operator.

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 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.

test function:

You can use multiple arguments represented by *args and multiple keywords represented by **kwargs and passing to a function:

def test(*args, **kwargs):
    print('arguments are:')
    for i in args:
        print(i)

    print('\nkeywords are:')
    for j in kwargs:
        print(j)

Example:

Then use any type of data as arguments and as many parameters as keywords for the function. The function will automatically detect them and separate them to arguments and keywords:

a1 = "Bob"      #string
a2 = [1,2,3]    #list
a3 = {'a': 222, #dictionary
      'b': 333,
      'c': 444}

test(a1, a2, a3, param1=True, param2=12, param3=None)

Output:

arguments are:
Bob
[1, 2, 3]
{'a': 222, 'c': 444, 'b': 333}

keywords are:
param3
param2
param1