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

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?