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 ?

• Plopp: @Graipher Well, I do believe this is exactly what I asked, my code is working but I'd like improvement. Maybe it's not clear tho, I'll rewrite a bit
• Plopp: @Graipher Oh I didn't though I needed a working example as I wasn't expecting any code back but more a general idea, I've edited it :)
• Mast: We got a very good meta thread about how to improve question quality if you're interested :-)
• jfs: depending on the specific case, a simpler cli interface might be preferable: just pass one or two files as positional parameters (raise ArgumentError if given files in a wrong format). Using docopt, you would specify the alternatives on two/three lines. Here's a variant with the two line usage: command <file.ext1> $ command <file1.ext2> [<file2.ext2>]

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)