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)

Answer 2

A way to do this in Python 3.4 is to use the argparse.FileType class. Make sure to close the input stream when you are finished. This is also useful because you can use the pseudo-argument '-' for STDIN/STDOUT. From the documentation:

FileType objects understand the pseudo-argument '-' and automatically convert this into sys.stdin for readable FileType objects and sys.stdout for writable FileType objects

Example:

#!/usr/bin/env python3

import argparse

if __name__ == '__main__':
  parser = argparse.ArgumentParser()
  parser.add_argument('--infile', type=argparse.FileType('r', encoding='UTF-8'), 
                      required=True)
  args = parser.parse_args()
  print(args)
  args.infile.close()

And then when ran...

• Without argument:

  $ ./stack_overflow.py
  usage: stack_overflow.py [-h] --infile INFILE
  stack_overflow.py: error: the following arguments are required: --infile
  

• With nonexistent file:

  $ ./stack_overflow.py --infile notme
  usage: stack_overflow.py [-h] --infile INFILE
  stack_overflow.py: error: argument --infile: can't open 'notme': [Errno 2] No such file or directory: 'notme'
  

• With an existing file:

  $ ./stack_overflow.py --infile ./stack_overflow.py
  Namespace(infile=<_io.TextIOWrapper name='./stack_overflow.py' mode='r' encoding='UTF-8'>)
  

• Using '-' for STDIN:

  $ echo 'hello' | ./stack_overflow.py --infile -
  Namespace(infile=<_io.TextIOWrapper name='<stdin>' mode='r' encoding='UTF-8'>)