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pythonargparse

ArgumentParser: Optional argument with optional value

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.

Solution

  • 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.