ArgumentParser: Optional argument with optional value

Question

If I have an optional argument with optional argument value, is there a way to validate if the argument is set when the value is not given?

For instance:

parser = argparse.ArgumentParser()
parser.add_argument('--abc', nargs='?')
args = parser.parse_args()

Would correctly give me:

optional arguments:
    --abc [ABC]

How do I distinguish between 1 and 2 below?

1. '' => args.abc is None
2. '--abc' => args.abc is still None
3. '--abc something' => args.abc is something

...

Update:

Found a trick to solve this problem: you can use "nargs='*'" instead of "nargs='?'". This way #1 would return None, and #2 would return an empty list. The downside is this will allow multiple values for the arguments to be accepted too; so you'd need to add a check for it if appropriate.

Alternatively you can also set a default value for the argument; see answer from chepner and Anand S Kumar.

Answer 1

With nargs='?', you can supply both a default and const.

In [791]: parser=argparse.ArgumentParser()    
In [792]: parser.add_argument('--abc', nargs='?', default='default', const='const')

If the argument is not given it uses the default:

In [793]: parser.parse_args([])
Out[793]: Namespace(abc='default')

If given, but without an argument string, it uses the const:

In [794]: parser.parse_args(['--abc'])
Out[794]: Namespace(abc='const')

Otherwise it uses the argument string:

In [795]: parser.parse_args(['--abc','test'])
Out[795]: Namespace(abc='test')

In [796]: parser.print_help()
usage: ipython3 [-h] [--abc [ABC]]

optional arguments:
  -h, --help   show this help message and exit
  --abc [ABC]

Answer 2

Use a different default value for the option. Compare

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--abc', nargs='?', default="default")
>>> parser.parse_args()
Namespace(abc='default')
>>> parser.parse_args(['--abc'])
Namespace(abc=None)
>>> parser.parse_args(['--abc', 'value'])
Namespace(abc='value')

I'm not sure how you would provide a different value for when --abc is used without an argument, short of using a custom action instead of the nargs argument.