Command Line Arguments

Example 1: Generate Help Argument and Message

Consider the code sample below:

import argparse

parser = argparse.ArgumentParser(description='A test program.')
args = parser.parse_args()

$ ./test.py -h
$ ./test.py --help

A test program.

optional arguments:
  -h, --help  show this help message and exit

Example 2: Handle a String Argument

import argparse

parser = argparse.ArgumentParser(description='A test program.')

parser.add_argument("print_string", help="Prints the supplied argument.")

args = parser.parse_args()

print(args.print_string)

usage: test.py [-h] [print_string]

A test program.


positional arguments:

  print_string  Prints the supplied argument.


optional arguments:

  -h, --help    show this help message and exit

To define and parse optional arguments, you can use “–” (double dash) and change their default values using the “default” argument.

import argparse

parser = argparse.ArgumentParser(description='A test program.')

parser.add_argument("--print_string", help="Prints the supplied argument.", default=”A random string.”)

args = parser.parse_args()

print(args.print_string)

$ ./test.py --print_string My-Test-String
My-Tesr

You can also define shorthand versions of the argument using “-” (single dash) to reduce verbosity.

import argparse

parser = argparse.ArgumentParser(description='A test program.')

parser.add_argument(“-p”, "--print_string", help="Prints the supplied argument.", default=”A random string.”)

args = parser.parse_args()

print(args.print_string)

$ ./test.py -p Test-String

Example 3: Handle an Integer Argument

import argparse

parser = argparse.ArgumentParser(description='A test program.')

parser.add_argument("-p", "--print_string", help="Prints the supplied argument.", type=int)

args = parser.parse_args()

print(args.print_string)

usage: test.py [-h] [-p PRINT_STRING]

test.py: error: argument -p/--print_string: invalid int value: 'LinuxHint.com'

Example 4: Handle True and False Toggles

import argparse

parser = argparse.ArgumentParser(description='A test program.')

parser.add_argument("-p", "--print_string", help="Prints the supplied argument.", action="store_true")

args = parser.parse_args()

print(args.print_string)

Example 5: Treat Argument Values as List

If you want to get multiple values at once and store them in list, you need to supply “nargs” keyword in following format:

import argparse

parser = argparse.ArgumentParser(description='A test program.')

parser.add_argument("-p", "--print_string", help="Prints the supplied argument.", nargs='*')

args = parser.parse_args()

print(args.print_string)

$ ./test.py -p “a” “b”

['a', 'b']

Example 6: File as command line argument for argparse - error message if argument is not valid

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)

Example 7: File Extension Checking - argsparse

import argparse
import os.path

parser = argparse.ArgumentParser()

def file_choices(choices,fname):
    ext = os.path.splitext(fname)[1][1:]
    if ext not in choices:
       parser.error("file doesn't end with one of {}".format(choices))
    return fname

parser.add_argument('fn',type=lambda s:file_choices(("csv","tab"),s))

parser.parse_args()

Example 8: Require either of two arguments using argparse

import argparse

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('--foo',action=.....)
group.add_argument('--bar',action=.....)
args = parser.parse_args() 

If you need some check that is not provided by the module you can always do it manually:

pa = argparse.ArgumentParser()
...
args = pa.parse_args()

if args.foo is None and args.bar is None:
   pa.error("at least one of --foo and --bar required")

Example 8: If arg is A, do this, if B do that, if none of the above show help and quit

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("a")
args = parser.parse_args()

if args.a == 'magic.name':
    print 'You nailed it!'

adding to it:

...
parser.add_argument("a", nargs='?', default="check_string_for_empty")
...
if args.a == 'check_string_for_empty':
    print 'I can tell that no argument was given and I can deal with that here.'
elif args.a == 'magic.name':
    print 'You nailed it!'
else:
    print args.a

Or try the following:

Here's the way I do it with argparse (with multiple args):

parser = argparse.ArgumentParser(description='Description of your program')
parser.add_argument('-f','--foo', help='Description for foo argument', required=True)
parser.add_argument('-b','--bar', help='Description for bar argument', required=True)
args = vars(parser.parse_args())

args will be a dictionary containing the arguments:

if args['foo'] == 'Hello':
    # code here

if args['bar'] == 'World':
    # code her