Pythonic way for adding parameters to argparse based on another parameter

Question

In the main function, I have a parser which validates optional inputs:

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument('--platform',required=True)
    parser.add_argument('--foo')
    parser.add_argument('--bar')
    parser.add_argument('--baz')
    parser.parse_args()

The above snippet is an example which only works when I supply --platform and either --foo,--bar or --baz.

This code is used by different components, let's call them components A, B and C.

Component A actually only specifies --foo and --bar:

python script.py --platform A --foo first_example --bar first_example

Component B actually only specifies --bar and --baz:

python script.py --platform B --bar second_example --baz second_exmaple

Component C actually only specifies --baz:

python script.py --platform C --baz third_example

As I introduce more components, which provide different arguments, the number of arguments I have to add to the parser increases. The above is just an example and I am currently dealing with 20 or so arguments (will likely be more in the future).

I have been thinking about having a configuration file (.yaml) where I define which arguments each component needs:

# parameters.yaml
A:
  - foo
  - bar

B:
  - bar
  - baz

C:
  - baz

I would like to simplify the main function to look at the --platform argument and, based on which platform has been passed as argument, read the configuration and add additional arguments to the parser.

Here's what I have tried:

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument('--platform',required=True)
    
    # Read from .yaml file
    with open('parameters.yaml') as parameter_file:
        parameters = yaml.safe_load(parameter_file)
    
    for argument in parameters[sys.argv[sys.argv.index('--platform') + 1]]:
        parser.add_argument(
           '--' + argument
    )

    parser.parse_args()

Calling the function:

python script.py --platform C --baz third_example

the above code works but I am looking for other Pythonic solutions as I am a beginner with Python. I don't really like having to look at sys.argv to determine what --platform has been specified. Are there any better solutions to this problem?

Answer 1

With subparsers as sub-commands:

import argparse

def run_command(parser, args):
    if args.command == 'A':
        print(args)
    elif args.command == 'B':
        print(args)
    elif args.command == 'C':
        print(args)

parser = argparse.ArgumentParser(
    prog='PROG', 
    epilog="See '<command> --help' to read about a specific sub-command."
)
subparsers = parser.add_subparsers(dest='command', help='Sub-commands')

A_parser = subparsers.add_parser('A', help='Platform A')
A_parser.add_argument("--foo")
A_parser.add_argument('--bar')
A_parser.set_defaults(func=run_command)

B_parser = subparsers.add_parser('B', help='Platform B')
B_parser.add_argument('--bar')
B_parser.add_argument('--baz')
B_parser.set_defaults(func=run_command)

C_parser = subparsers.add_parser('C', help='Platform C')
C_parser.add_argument('--baz')
C_parser.set_defaults(func=run_command)

args = parser.parse_args()
if args.command is not None:
    args.func(parser, args)
else:
    parser.print_help()

This generates:

~ python args.py -h
usage: PROG [-h] {A,B,C} ...

positional arguments:
  {A,B,C}     Sub-commands
    A         Platform A
    B         Platform B
    C         Platform C

optional arguments:
  -h, --help  show this help message and exit

See '<command> --help' to read about a specific sub-command.

and

~ python args.py B -h
usage: PROG B [-h] [--bar BAR] [--baz BAZ]

optional arguments:
  -h, --help  show this help message and exit
  --bar BAR
  --baz BAZ