When I run the following program, I expect args.run to be 'foo' if no argument is specified on the command-line.

parser = argparse.ArgumentParser()

parser.add_argument('foo', default='bar')

args = parser.parse_args()

$ ./test
usage: test [-h] foo
test: error: the following arguments are required: foo

However if I specify nargs='*' this error goes away.

Maybe I'm missing something obvious, but this seems non-intuitive to me.

Sorry, I meant to say args.foo should be 'bar'.

Providing a default value does not imply setting nargs, so you need to specify both. I think it's not clear what nargs would default to, if you wanted to change the behavior if default= is supplied.. In your example you probably want nargs='?', not '*'.

Do you think it's unreasonable/unintuitive to infer nargs from the default value?

Would you infer it to be ‘?’ or ‘*’?

I’m not completely opposed, but I haven’t thought it through or looked at the code.

I think your original suggestion makes sense: '?'

My intuition is that nargs helps argparse discern whether it's dealing with single or multiple values. That may not be what was intended.

There probably shouldn't be multiple ways of indicating an argument is optional. It seems that required= and nargs='?'|'*' combinations could lead to conflicting requirements.