In python argparse, is there a use case for nargs=1?

Question

It seems as though it would always make more sense to use the default action of store without specifying nargs so the output is always as expected, instead of sometimes being a list and sometimes not. I'm just curious if I missed something..

example

>>> import argparse                                                                                                                     
>>> parser = argparse.ArgumentParser()                                                                                                  
>>> parser.add_argument('--foo')                                                                                                        
_StoreAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)                                                                                                                                      
>>> parser.add_argument('--bar', nargs=1)                                                                                               
_StoreAction(option_strings=['--bar'], dest='bar', nargs=1, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>> parser.parse_args('--foo 1 --bar 1'.split())                                                                                        
Namespace(bar=['1'], foo='1')                                                                                                           
>>> parser.parse_args('')                                                                                                               
Namespace(bar=None, foo=None)                                                                                                           

Answer 1

Both the default nargs=None and nargs=1 expect one value, but nargs=1 will put it in a list, e.g.

Namespace(foo='test')
Namespace(foo=['test'])

nargs=3 will require 3 values, and put them in a list as well. * and + also put the values in a list.

From the docs, under nargs=N: https://docs.python.org/3/library/argparse.html#nargs

Note that nargs=1 produces a list of one item. This is different from the default, in which the item is produced by itself.

nargs=1 is just an instance of nargs=n. It's not a special case.

For you, as an argparse user, you probably don't need to use 1. In fact to me it's a sign of a novice - unless there's a clear need for the list in the output. I can imagine, for example, building a parser that programmatically sets n to some number, may be 3, maybe 5, maybe 1, and expects the result to always be a list.