Variable argument count plays badly with choices.

Example:
========
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('choices', nargs='*', default='a', choices=['a',
'b', 'c'])
>>> args = parser.parse_args()
>>> print type(args.choices)
<type 'str'>
>>> args = parser.parse_args(['a'])
>>> print type(args.choices)
<type 'list'>


If the user specifies the value on the command line then a list is used, but if the value comes from the default a string is used.
Unfortunately, changing default to a list value gives an error:
error: argument choices: invalid choice: ['a'] (choose from 'a', 'b',
'c')

Additionally, this means it is also not possible to use default=['a', 'c']. 

The current workaround is to create a custom type:

def my_type(string):
    if string not in ['a', 'b', 'c']:
        raise TypeError
    return string

Maybe it will sound strange, but what is this task REALLY about? I mean - I can see two problems here, but no clear information about which problem is a real problem and - if it is - what is the expected behavior.

Problems I can see are:
1) Type of returned value changes depending on the value source (list for user provided or list for default)
2) It's impossible to set list as 'default'

I understand that this task concentrates on 1st one, however changing behavior described in 2nd would - in my oppinion - fix 1st one too - am I right? User would "define" the returned type by defining the 'default' as a list or string. 

But - if we assume that we only discuss 1st problem here - what is the expected behavior? Provided workaround is suggesting that we expect string, but - basing on current nargs behavior and "intuition" - I'd rather expect to get a list, when I define nargs with "*" or "+". This sounds "natural" for me.

Could someone explain me the problem and the expected behavior in a clear way?

Sorry if I'm not clear, but it's my first post here and maybe I've missed something in the description or I assume something that is not true (especially that the task is quite old... ;) ).

Of course I've made a mistake:

"list for user provided or list for default"

should be:

"list for user provided or STRING for default"

The real issue is that the choices flag does not work with a default flag and * nargs.

The following works as expected:
>>> parser.add_argument('chosen', nargs='*', default=['a'])
>>> print(parser.parse_args())
Namespace(chosen=['a'])
>>> print(parser.parse_args(['a', 'b']))
Namespace(chosen=['a', 'b'])

Introducing a choices constraint breaks down when using the defaults:
>>> parser.add_argument('chosen', nargs='*', default=['a'], choices=['a', 'b'])
>>> print(parser.parse_args(['a']))
Namespace(chosen=['a'])
>>> print(parser.parse_args())
error: argument chosen: invalid choice: ['a'] (choose from 'a', 'b')

I would expect instead to have Namespace.chosen populated with the default list as before, but the choices constraint check does not validate correctly.

I think that changing the choices constraint logic to iterate over the default values if nargs results in a list would be a possible solution.