The type of the argument should be string (which is default anyway). So make it like this:

parser = argparse.ArgumentParser()
parser.add_argument('filename')
args = parser.parse_args()
with open(args.filename) as file:
# do stuff here

Take a look at the documentation: https://docs.python.org/3/library/argparse.html#type

import argparse


parser = argparse.ArgumentParser()
parser.add_argument('file', type=argparse.FileType('r'))
args = parser.parse_args()


print(args.file.readlines())

This implementation allows the "file name" parameter to be optional, as well as giving a short description if and when the user enters the -h or --help argument.

parser = argparse.ArgumentParser(description='Foo is a program that does things')
parser.add_argument('filename', nargs='?')
args = parser.parse_args()


if args.filename is not None:
print('The file name is {}'.format(args.filename))
else:
print('Oh well ; No args, no problems')

In order to have the file closed gracefully, you can combine argparse.FileType with the "with" statement

# ....


parser.add_argument('file', type=argparse.FileType('r'))
args = parser.parse_args()


with args.file as file:
print file.read()

--- update ---

Oh, @Wernight already said that in comments

I'll just add the option to use pathlib:

import argparse, pathlib


parser = argparse.ArgumentParser()
parser.add_argument('file', type=pathlib.Path)
args = parser.parse_args()


with args.file.open('r') as file:
print(file.read())

TL; DR

parser.add_argument(
'-f', '--file',
help='JSON input file',
type=argparse.FileType('r'),
)

Description

Simple command line script to reformat JSON files

reformat-json \
-f package.json \
--indent=2 \
--sort-keys \
--output=sorted_package.json

can be code in Python as follows

#!/usr/bin/env python3


import argparse, json, sys


EXIT_SUCCESS = 0
EXIT_FAILURE = 1


def main():
parser = argparse.ArgumentParser()


parser.add_argument(
'-f', '--file',
help='JSON input file',
type=argparse.FileType('r'),
)


parser.add_argument(
'-i', '--indent',
help='Non-negative integer indent level',
type=int
)


parser.add_argument(
'-o', '--output',
help='Write JSON into output file',
type=argparse.FileType('w'),
)


parser.add_argument(
'-s', '--sort-keys',
action='store_true',
help='Sort output JSON by keys',
)


args = parser.parse_args()


if not args.file:
parser.print_usage()
return sys.exit(EXIT_FAILURE)


gson = json.dumps(
json.load(args.file),
indent=args.indent,
sort_keys=args.sort_keys
)


args.file.close()


if args.output:
args.output.write(gson)
args.output.write('\n')
args.output.close()
else:
print(gson)


return sys.exit(EXIT_SUCCESS)


if __name__ == '__main__':
main()