Python argparse, positional arguments and subparsers

Question

I have the following snippet where I am using argparse with multiple subparsers

parser = argparse.ArgumentParser(description = "Setup the DB",
        add_help=False)
parser.add_argument('action', type = str,
        choices=['drop','populate','print','create','print-metadata'],
        help = "Specify an action", default = None)
subparsers = parser.add_subparsers()
drop_parser = subparsers.add_parser('drop',parents=[parser])
drop_parser.add_argument('-dataset-name',
        required=True,
        type = str, help = "Dataset Name",
        default = None)
.....
args = parser.parse_args()
.....

When I go to run this I get the following:

python .\populatedb.py drop -dataset-name foo
populatedb.py: error: invalid choice: 'foo' (choose from 'drop', 'populate', 'print', 'create', 'print-metadata')

I am wondering where the code above is going wrong. Note that the "action" argument is a positional argument.

Thank you in advance. This is my first use of subparsers I am probably making an obvious mistake.

Thank you in advance.

Regards,

Ranga

Thanks for the quick reply. The argument is only meaningful for the action "drop" . For other options it is not meaningful hence I added it to the subparser. I left out the other subparsers for clarity. — LostInTheFrequencyDomain, Sep 12 '16 at 22:19
It feels like you might need to add 'action drop'. You can run python populatedb.py -h to see the sequence of arguments you need to pass. — Stanley Kirdey, Sep 12 '16 at 22:25
python .\populatedb.py -help gives the following : usage: populatedb.py {drop,populate,print,create,print-metadata} so that does not seem to be the issue. — LostInTheFrequencyDomain, Sep 12 '16 at 22:30
But generally you don't need to `add_action` _and_ add a subparser with the same name ... — mgilson, Sep 12 '16 at 23:02
@mgilson I removed the add_argument from the parent parser and now I get further; however, how would I know which subparser got invoked in that case? — LostInTheFrequencyDomain, Sep 12 '16 at 23:23
The answer is here http://stackoverflow.com/questions/8250010/argparse-identify-which-subparser-was-used — LostInTheFrequencyDomain, Sep 12 '16 at 23:29

mgilson · Accepted Answer · 2016-09-12T23:31:32.530

The generally accepted thing to do here is to not have an argument for the subparser -- it is it's own argument provided by argparse:

parser = argparse.ArgumentParser(description = "Setup the DB",
        add_help=False)
subparsers = parser.add_subparsers()
drop_parser = subparsers.add_parser('drop',parents=[parser])
drop_parser.add_argument('-dataset-name',
        required=True,
        type = str, help = "Dataset Name",
        default = None)
.....
args = parser.parse_args()

Now you don't necessarily know which parser was selected but don't worry, there's a built-in mechanism for this as well. One common use-case is having one function that should be invoked for each subparser...

parser = argparse.ArgumentParser(description = "Setup the DB",
        add_help=False)
subparsers = parser.add_subparsers()

def drop_parser_handler(args):
    ...

drop_parser = subparsers.add_parser('drop',parents=[parser])
drop_parser.add_argument('-dataset-name',
        required=True,
        type = str, help = "Dataset Name",
        default = None)
drop_parser.set_defaults(func=drop_parser_handler)
.....

args = parser.parse_args()
args.func(args)

Of course you don't need to do anything this complex -- You could just set a constant value:

drop_parser.set_defaults(subparser_name='drop')

Thanks! That helped. – LostInTheFrequencyDomain Sep 12 '16 at 23:31 — LostInTheFrequencyDomain, Sep 12 '16 at 23:31

Python argparse, positional arguments and subparsers

1 Answers1