How to add command line arguments in Python?

Introduction..

Python has a very powerful argparse module which provides functions for parsing command line arguments. If we want to get the user input from the OS command line without a lot of interaction or code a program that accepts parameters from the command line e.g. Provide a URL to parse or accepts the file to upload to a S3 bucket then argparse can be used with minimal effort.

Basic Usage

• Define the arguments that your code is going to accept.

• Call the argument parser to return the results object.

• Use the arguments.

In short, the structure of argument parser looks some thing like below.

def main( parameters):
<< Logic here >>

if __name__ == '__main__':
<< 1. Define argument parser >>
<< 2. Parse the arguements >>
<< 3. Validation >>
<< 4. call main (parameters) >>

The main function knows what the entry point of our code is. The __name__ == '__main__' section is executed only if the code is called directly.

• Create a program which will accept only one argument - tennis players as string.

import argparse

def get_args():
""" Function : get_args
1. metavar - Provide a hint to the user about the data type.
- By default, all arguments are strings.

2. type - The actual Python data type
- (note the lack of quotes around str)

3. help - A brief description of the parameter for the usage

"""

parser = argparse.ArgumentParser(
description='Example for Two positional arguments',
formatter_class=argparse.ArgumentDefaultsHelpFormatter)

# Adding our first argument player name of type string
metavar='player',
type=str,
help='Tennis Player')

return parser.parse_args()

# define main
def main(player):
print(f" *** The {player} had won 20 grandslam titles.")

if __name__ == '__main__':
args = get_args()
main(args.player)

a) Now when you execute this program from the command line without passing any parameters i.e. If given nothing, it will print a brief usage statement about the proper way to invoke the program.

In [3]: run <>.ipynb
usage: ipython [-h] player
ipython: error: the following arguments are required: player
An exception has occurred, use %tb to see the full traceback.

b) If we provide more than one argument, it complains again.The program complains about getting a second argument that has not been defined.

c) Only when we give the program exactly one argument will it run

2. Create a program which will accept only two arguments - tennis players as string and grand slamt titles won by the player as integer.

Example

import argparse

def get_args():
""" Function : get_args
1. metavar - Provide a hint to the user about the data type.
- By default, all arguments are strings.

2. type - The actual Python data type
- (note the lack of quotes around str)

3. help - A brief description of the parameter for the usage

"""

parser = argparse.ArgumentParser(
description='Example for Two positional arguments',
formatter_class=argparse.ArgumentDefaultsHelpFormatter)

# Adding our first argument player name of type string
metavar='player',
type=str,
help='Tennis Player')

# Adding our second argument player titles of type integer/number.
metavar='titles',
type=int,
help='Tennis Player Grandslam Titles')

return parser.parse_args()

# define main
def main(player, titles):
print(f" *** The {player} had won {titles} grandslam titles.")

if __name__ == '__main__':
args = get_args()
main(args.player, args.titles)



Now open your terminal and execute the program. If the arguments are not passed, the script throw back error with clear message.

Output

<<< python test.py
usage: test.py [-h] player titles
test.py: error: the following arguments are required: player, titles

<<< python test.py federer 20
*** The federer had won 20 grandslam titles.