How to make an optional value for argument using argparse?

Question

I am creating a python script where I want to have an argument that manipulates how many search results you get as output. I've currently named the argument --head. This is the functionality I'd like it to have:

1. When --head is not passed at the command line I'd like it to default to one value. In this case, a rather big one, like 80

2. When --head is passed without any value, I'd like it to default to another value. In this case, something limited, like 10

3. When --head is passed with a value, I'd like it to store the value it was passed.

Here is some code describing the problem:

>>> import argparse >>> parser = argparse.ArgumentParser() >>> parser.add_argument('-h',                         '--head',                         dest='size',                         const=80,                         default=10,                         action="I don't know",                         help='Only print the head of the output') >>> # OFC, that last line will fail because the action is uknown, ... # but here is how I'd like it to work ... parser.parse_args(''.split()) Namespace(size=80) >>> parser.parse_args('--head'.split()) Namespace(size=10) >>> parser.parse_args('--head 15'.split()) Namespace(size=15) 

I know I probably can write a custom action for this, but I first want to see if there is any default behaviour that does this.

Answer 1

After a little more reading in the documentation I found what I needed: nargs='?'. This is used with the store action, and does exactly what I want.

Here is an example:

>>> import argparse >>> parser = argparse.ArgumentParser() >>> parser.add_argument('--head',                         dest='size',                         const=10,                         default=80,                         action='store',                         nargs='?',                         type=int,                         help='Only print the head of the output') >>> parser.parse_args(''.split()) ... Namespace(size=80) >>> parser.parse_args('--head'.split()) ... Namespace(size=10) >>> parser.parse_args('--head 15'.split()) ... Namespace(size=15) 

Source: http://docs.python.org/3/library/argparse.html#nargs