Python Optparse and Argparse Example

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Argparse module was in Python version 3.2 and up. It superseded optparse module. Inkscape 0.92 extension program inkex.py file was written many years ago with optparse module. I read several articles trying to understand how to use optparse and argparse modules. An example discussed in this blog post is very helpful.

The article has an example program designed with two options. Run the program on command line with options and arguments shown on that blog, and it shows how optparse module works behind scene.

Following code is a rewrite of the example with argparse. You can execute similar commands shown below and compare the results. Here is the link to the code in github: github repo. 

#!/usr/bin/env python3
# argparse_example.py
# This is a version using argparse
# revised by GHZ on 2/2019

import argparse

def main():

    parser = argparse.ArgumentParser(
        usage = 'usage: %(prog)s [options] filename'
    )

    parser.add_argument('-x', '--xhtml',
        action = 'store_true', dest = 'xhtml_flag',
        default = False,
        help = 'create a XHTML template instead of HTML'
        )

    parser.add_argument('-c', '--cssfile',
        action = 'store',
        dest = 'cssfile',
        default = 'style.css',
        help = 'CSS file to link',
    )

    parser.add_argument('filename', nargs='?',  # '*' for multiple files
        action = 'store',
        help = 'the name of file to be processed'
    )

    parser.add_argument('-V', '--version',
        action='version', version='%(prog)s 1.0')

    args = parser.parse_args()
    print(args)
    print(args.filename)

if __name__ == '__main__':
    main()

Here are the bash commands to test code. 

python argparse_example.py -h
python argparse_example.py 
python argparse_example.py myfile.html
python argparse_example.py -x -c mystyle.css myfile.html
python argparse_example.py --xhtml --cssfile=mystyle.css myfile.html

The argparse module contains many other options. Here is a link to Python argparse Official Doc.

Argparse v.s. Optparse

The two modules have some obvious differences. Run the commands below to start the scripts. The options or arguments start after optparse_ex.py and argparse_ex.py. 

$python2 optparse_ex.py ...
$python argparse_ex.py ...

The optparse module separates command line arguments into options and args two groups. It calls parser.add_option methods to add options, and it does not need to add args to the parser beforehand. The argparse module has positional arguments and optional arguments. It calls parser.add_argument method to add arguments.

Argparse module seems to widely used in many open source projects, but newer third party module (e.g. Click) is also available.

Click Package

If you write the example as a click app, the code looks like this. The click package uses python decorators. The code logic seems to be a little clearer. 

import click

@click.command()
@click.option('--xhtml', 
              '-x', 
              default=False, 
              help='create a XHTML template instead of HTML')
@click.option('--cssfile', 
              '-c', 
              default='style.css',
              help="CSS file to link")
@click.argument('file')
def main(xhtml, cssfile, file):
    print("xhtml option is: \n    ", xhtml)
    print("cssfile option is : \n    ", cssfile)
    print("file is : \n    ", file)

if __name__ == '__main__':
    main()

The bash commands to test code will be a little different than above.

python click_example.py -h
python click_example.py
python click_example.py myfile.html
python click_example.py -x -c mystyle.css myfile.html # error
python click_example.py -x True -c mystyle.css myfile.html
python click_example.py --xhtml True -c mystyle.css myfile.html
python click_example.py --xhtml True --cssfile mystyle.css myfile.html

The weird part of the click example code is that the defined main function has three positional arguments. The decorator changes the function definition to be without arguments. When you call the main fuction, it is simply main() without any arguments. Normally that is not a good decorator practice, but this is an exception and it works well.