How to change argparse option action vs. type behavior based on argument presence?

Question

I'm trying to implement the following behavior for the CLI for my Python script:

if the -o option is not present, the script does not write to a file
if the -o option is present without an argument, the script writes to a default filename
if the -o option is present with an argument, the script writes to a filename supplied by the user

$ python myscript.py               # does not write to file
$ python myscript.py -o            # writes to default.txt
$ python myscript.py -o myfile.txt # writes to myfile.txt

Is it possible to achieve this with argparse? Here's what I've tried when building my parser:

parser.add_argument('-o', '--output', action='store_true', type=str, default=False, required=False, help="...")

wcarhart · Accepted Answer · 2019-09-19 19:09:31Z

1

I learned that this is possible, but you need to use the const and nargs options.

parser.add_argument('-o', '--output', const='default.txt', nargs='?', type=str, default=False, required=False, help="...")

1 Comment

chepner Over a year ago

Nice: I wasn't aware of const being used apart from store_const or append_const actions.

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