How to handle dependent arguments using argparse?

Question

I am trying to configure argparse so that only the following argument combinations are valid:

--flagA --argument1 val1 --argument2 val2 --argument3 val3
--flagB --argument1 val1

So far, I have written the following piece of code however it doesn't do the trick as both --argument2 and --argument3 are optional:

required_args = arg_parser.add_argument_group()
required_args.add_argument('--argument1', required=True)

# Optional arguments
optional_args = arg_parser.add_argument_group()
optional_args.add_argument('--argument2', required=False)
optional_args.add_argument('--argument3', required=False)

# Flags
flag_group = arg_parser.add_mutually_exclusive_group(required=True)
flag_group.add_argument('--flagA', dest='flag', action='store_const', const="flagA")
flag_group.add_argument('--flagB', dest='flag', action='store_const', const="flagB")

args = vars(arg_parser.parse_args())

With this setting, --flagA --argument1 val1 is also but it shouldn't be.

I know that I can process the Namespace generated after calling .parse_args() and check whether --argument2 and --argument3 are provided when --flagA is passed however I am looking for a more natural way to achieve this.

Essentially, the definition should look like the one below:

[-h] --argument1 ARGUMENT1 (--flagA --argument2 ARGUMENT2 --argument3 ARGUMENT3 | --flagB)

Mutually Exclusive Groups is the only co-occurrence tool in argparse. It's a simple xor. If that doesn't work for you, use the post-parsing testing. There's nothing 'natural' about trying to use argparse for everything. — hpaulj
– hpaulj, Commented Nov 7, 2019 at 18:57

Zeph · Accepted Answer · 2019-11-07 21:05:02Z

2

Have you considered using a subparser for the two flags?

something along the lines of

arg_parser = argparse.ArgumentParser()

subparser = arg_parser.add_subparsers(dest="flag", required=True)

flag_a = subparser.add_parser('flagA')
flag_a.add_argument('--argument1', required=True)
flag_a.add_argument('--argument2', required=True)
flag_a.add_argument('--argument3', required=True)

flag_b = subparser.add_parser('flagB')
flag_b.add_argument('--argument1', required=True)

Hope this helps

edited Nov 7, 2019 at 21:05

answered Nov 7, 2019 at 18:45

Zeph

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3 Comments

Tokyo Over a year ago

This does not work properly. Even python myscript.py --flagB --argument1 val1 does not pass.

chepner Over a year ago

flagA and flagB aren't options any more: python myscript.py flagB --argument val1.

Zeph Over a year ago

This version works, but the flags don't have the "--" prefix when you call them since that prefix causes the arg parser to identify the argument as a new argument rather than the input for the subcommand. looking for a workaround now.

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