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

Answer 2

Using assert.

You can add assert if you want to forbid some parameters. The example:

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

if args.platform == 'A':
    assert args.baz is None, "no argument baz for the platform A"
...

Using del

Excluding the parameters, that you don't need, with del:

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

if args.platform == 'A':
    del args.baz
...

---

I wouldn't recommend you to use argparse and sys.argv in the same script, because they are for the same purpose in your case. It is better to choose one of them.

argparge parses your arguments before your script knows the values. Thus you can't change the list of parameters depending on the value of one of them. So if you want to keep argparse, you have to add the whole set of arguments with .add_argument and to do something extra below when the values are gotten.