Argparse optional argument with different default if specified without a value

Question

Consider three different runs of a program:

python3 prog.py
python3 prog.py --x
python3 prog.py --x 2

Is it possible to use argparse<

Answer 1

If you are happy to do a little post-processing, you can get it:

sentinel = object()
parser.add_argument('--x', nargs='?', type=int, default=sentinel)
args = parser.parse_args()
if args.x is sentinel:
    args.x = None
elif args.x is None:
    args.x = 1

However, it is bending the tool in a bit of a strange way. You may want to consider instead the count action, which is commonly used to specify verbosity levels, for example (-v, -vv, -vvv).

Answer 2

nargs'?' with a const parameter handles this 3-way input nicely..

In [2]: parser = argparse.ArgumentParser()
In [3]: parser.add_argument('-x','--x', nargs='?', type=int, const=1)
...
In [4]: parser.parse_args([])
Out[4]: Namespace(x=None)
In [5]: parser.parse_args(['-x'])
Out[5]: Namespace(x=1)
In [6]: parser.parse_args(['-x','2'])
Out[6]: Namespace(x=2)

I could have also given it a default parameter.

how to add multiple argument options in python using argparse?