Managaing Command Line Arguments in Python

I recently came across a program where we had to manage the command line arguments. And the easy way to go do that was using argparse package.

argparse is a Python module in the standard library that provides a mechanism for parsing command-line arguments. It makes it easy to write user-friendly command-line interfaces for your Python programs.

What can you do with it? Let me show you some of the capabilities.

Command Line Help

Following is a simple program with argparse.

import argparse

def main() -> None:
    parser = argparse.ArgumentParser(
                    prog='Sample Program',
                    description='Demonstrated Argparse Functionality',
                    epilog='Happy coding')
    parser.add_argument('-f', '--filename')           # positional argument
    parser.add_argument('-c', '--count')      # option that takes a value
    parser.add_argument('-v', '--verbose')  # on/off flag

    args = parser.parse_args()
    print(args.filename, args.count, args.verbose)

if __name__ == '__main__':
    main()

You can run this in as a python program. When you execute the following command, you will see

python main.py --help

usage: Sample Program [-h] [-f FILENAME] [-c COUNT] [-v VERBOSE]

Demonstrated Argparse Functionality

options:
  -h, --help            show this help message and exit
  -f FILENAME, --filename FILENAME
  -c COUNT, --count COUNT
  -v VERBOSE, --verbose VERBOSE

Happy coding

Required Arguments

You can make certain arguments as required like shown below.

 parser.add_argument(‘-f’, ‘–filename’, required=True)  

When you execute the program with out filename it will show you the message like below.

usage: Sample Program [-h] -f FILENAME [-c COUNT] [-v VERBOSE]
Sample Program: error: the following arguments are required: -f/--filename

Fixed Argument Values

Lets say filename has to be “text1.txt” or “text1.csv”. It can not be any value other than that. Then you can specify the choice to restrict the values like shown below.

parser.add_argument('-f', '--filename', required=True, choices=["text1.txt","text1.csv"])    

If you try to run the program with invalid choice, you will get the following error.

usage: Sample Program [-h] -f {text1.txt,text1.csv} [-c COUNT] [-v VERBOSE]
Sample Program: error: argument -f/--filename: invalid choice: 'somefile.txt' (choose from 'text1.txt', 'text1.csv')

Constant Values

Suppose I do not want to pass the count, and set a default count to 1.

parser.add_argument('-c', '--count', const=1,action='store_const') 

When the execute the program with the following command

python main.py -f text1.txt  -c  -v "verbose"

I see the values as

text1.txt 1 verbose

Notice that I am not passing argument after -c. It is a constant which is considered internally.

Closing Note

Argparse has a lot of features around command line arguments and execution. This is few of the ones which I noticed on the surface. There are more features like adding type safety to the arguments. I will be sure to post as I come across an interesting feature. Until then bye, see you around, and thanks for reading the blog.