Python CLI-Parser improvement for "one of those options"

Question

I was wondering if there was a better way to implement my cli parser.

For context: I need to pass to my script either a file in one of two formats or two files in a specific format.

It may be easier to understand like this:

file.ext1 OR file.ext2 OR (file_1.ext2 AND file_2.ext2)

---

I've used python argparse add_mutually_exclusive_group successfully and it looks like this:

import argparse
parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument(
    "--ext1",
    type    = __existant_file,
    metavar = "FILE",
    help    = "input file (format ext1)"
)
group.add_argument(
    "--ext2",
    type    = __existant_file,
    metavar = "FILE",
    help    = "input file (format ext2)"
)
group.add_argument(
    "--paired",
    nargs   = 2,
    type    = __existant_file,
    metavar = ("MATE_1", "MATE_2"),
    help    = "input file (two files format ext2)"
)

args = parser.parse_args()
print(args)

if args.ext1 is not None:
  file = args.ext1
elif args.ext2 is not None:
  file = args2.ext2
else:
  file = args.paired[0]
  file2 = args.paired[1]

Which is used as:

python script.py --ext1 file
OR
python script.py --ext2 file
OR
python script.py --paired file_1 file_2

Which is working but not really smooth. Do you have any lead of how I can improve the CLI parser ?

Answer 1

Okay, I found a better way

import argparse
parser = argparse.ArgumentParser()

class RequiredLen(argparse.Action):
        def __call__(self, parser, args, values, option_string=None):
            if not 1 <= len(values) <= 2:
                msg = f"argument {self.dest} requires 1 or 2 arguments"
                raise argparse.ArgumentTypeError(msg)
            setattr(args, self.dest, values)
# Just give me 1 or 2 files
parser.add_argument(
    "--paired",
    nargs    = '+',
    action   = RequiredLen,
    required = True,
    help     = "input file(s)"
)
# Specify either ext1 or ext2 format
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument(
    "--ext1",
    action = "store_const",
    dest   = "format",
    const  = "ext1",
    help   = "input is ext1 format"
)
group.add_argument(
    "--ext2",
    action = "store_const",
    dest   = "format",
    const  = "ext2",
    help   = "input is ext2 format"
)
args = parser.parse_args()
print(args)

This way I can later just use something like:

def process_ext1(l: list):
   print("ext1 file(s)")
   for i in l:
       print(i)

def process_ext2(l: list):
   print("ext2 file(s)")
   for i in l:
       print(i)

process_format = { 'ext1': process_ext1, 'ext2': process_ext2 }
process_format[args.format](args.paired)