Python argparse: Processing Certain Arguments Before Adding Others

Question

Using python's argparse library, I would like to process the first few command-line arguments and use them to generate a list of choices for the other command-line arguments.

How can I process the first few arguments without argparse complaining about extra arguments that it doesn't expect (which I plan to add later)?

For example, I have a script that gets the username and password from the command-line, uses those to access available properties of an API, and then uses that list to restrict the values of a third argument:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('username', help='Your username.')
parser.add_argument('password', help='Your password.')
args = parser.parse_args() # Error here because a third argument exists on the command-line

response = requests.get(
    url='https://api.example.com/properties',
    auth=(args.username, args.password)
)

parser.add_argument(
    'property',
    choices=response.json()['properties'], # Validates the input
    help='The property you want to access.'
)
args = parser.parse_args()

I know I can just add all the arguments at once, and then validate the third argument myself manually, but I'm wondering if there's a way to natively do what I'm asking in the argparse library?

Answer 1

parser = argparse.ArgumentParser()
parser.add_argument('username', help='Your username.')
parser.add_argument('password', help='Your password.')
parser.add_argument(
    'property',
    help='The property you want to access.'
)
args = parser.parse_args() 

response = requests.get(
    url='https://api.example.com/properties',
    auth=(args.username, args.password)
)
allowed_properties = response.json()['properties']
if args.property not in allowed_properties:
    parser.error('property not in allowed properties')

choices is not a complicated test. I'd recommend parsing all inputs, and then testing for this special case. Overall I think gives you better control, and better help and error display.

The parse_known_args approach is good to learn and occasionally use, but I don't think it's that great for this case. You don't get any extra points for embedding all argument testing in the parser.