Use argparse to define an option that takes a string consisting of any letters from a given set

Argparse does not have a built-in argument type that supports what you want, but it can be extended with a custom argument type to add support. For example,

from argparse import ArgumentTypeError
def myargumentconverter(arg):
    allowed = 'ARLQTNZCFXPB'
    for c in arg:
        if c not in allowed:
            raise ArgumentTypeError(f'{c} is not among "{allowed}"', help='Control Options')
    return arg
parser.register('type', 'options type', myargumentconverter)
parser.add_argument('-o', '--options', type='options type')

That will validate the argument against your string of allowed options, and store the whole string if it passes. If you prefer, it could perform additional validations, and / or it could return a different value to store in the argument namespace, such as a list of the individual options.

The code you wrote expects the user to enter -o A -o C .

If accepting args like BCA and ABC is a Requirement for your app, then you might prefer to do custom app-specific error checking outside of what argparse offers, and raise a diagnostic error upon seeing a letter not in "ABCFLNPQRTXZ" .

One big advantage of using argparse is you get instructional --help "for free". That won't be the case if there's a project requirement to support the example inputs you mentioned.