File as command line argument for argparse - error message if argument is not valid

Question

I am currently using argparse like this:

import argparse
from argparse import ArgumentParser

parser = ArgumentParser(description="ikjMatrix multiplication")
parser.add_argument("-i", dest="filename", required=True,
    help="input file with two matrices", metavar="FILE")
args = parser.parse_args()

A, B = read(args.filename)
C = ikjMatrixProduct(A, B)
printMatrix(C)

Now I would like to note, that the argument of -i should be a readable file. How can I do that?

I've tried adding type=open, type=argparse.FileType('r') and they worked, but if the file is not valid, I would like to get an error message. How can I do that?

Answer 1

It's pretty easy actually. You just need to write a function which checks if the file is valid and writes an error otherwise. Use that function with the type option. Note that you could get more fancy and create a custom action by subclassing argparse.Action, but I don't think that is necessary here. In my example, I return an open file handle (see below):

#!/usr/bin/env python

from argparse import ArgumentParser
import os.path


def is_valid_file(parser, arg):
    if not os.path.exists(arg):
        parser.error("The file %s does not exist!" % arg)
    else:
        return open(arg, 'r')  # return an open file handle


parser = ArgumentParser(description="ikjMatrix multiplication")
parser.add_argument("-i", dest="filename", required=True,
                    help="input file with two matrices", metavar="FILE",
                    type=lambda x: is_valid_file(parser, x))
args = parser.parse_args()

A, B = read(args.filename)
C = ikjMatrixProduct(A, B)
printMatrix(C)