Command Line and Variable Arguments in Python

Command Line Arguments

Command line arguments are input parameters which allow user to enable programs to act in a certain way like- to output additional information, or to read data from a specified source or to interpret the data in a desired format.

Python Command Line Arguments

Python provides many options to read command line arguments. The most common ways are -

• Python sys.argv
• Python getopt module
• Python argparse module

Python sys module

The sys module stores the command line arguments (CLA) into a list and to retrieve it, we use sys.argv. It’s a simple way to read command line arguments as string.

import sys

print(type(sys.argv))
print('The command line arguments are: ')
for i in sys.argv:
print(i)

One running above programme, our output will be something like -

Output

>python CommandLineOption.py H E L L O
<class 'list'>
The command line arguments are:
CommandLineOption.py
H
E
L
L
O

Python getopt module

The python getopt module parses an argument sequence like sys.argv and returns a sequence of options & argument pairs and a sequence of non-option arguments.

Syntax

getopt.getopt(args, options, [long_options])

where:

• Args – Argument list to be parsed

• Options – This is the string of option letters that the script wants to recognize.

• Long_options – This one is optional parameter, is a list of strings with the names of the long options, which should be supported.

The getopt module option syntax includes -

• -a
• -bval
• -b val
• --noarg
• --witharg=val
• --witharg val

Let’s understand the getopt module through an example -

Short form options

Below program takes two options, -a and –b with the second option requiring an argument.

import getopt
print (getopt.getopt(['-a', '-bval', '-c', 'val'], 'ab:c:'))

Result

([('-a', ''), ('-b', 'val'), ('-c', 'val')], [])

Long form option

The program takes two options, --noarg and –witharg and the sequence should be [‘noarg’, ‘witharg=’]

import getopt
print (getopt.getopt([ '--noarg', '--witharg', 'val', '--witharg2=another' ],'',[ 'noarg', 'witharg=', 'witharg2=' ]))

Result

([('--noarg', ''), ('--witharg', 'val'), ('--witharg2',
'another')], [])

Another example to demonstrate the usuage of geopt module -

import getopt
import sys
argv = sys.argv[1:]
try:
opts, args = getopt.getopt(argv, 'hm:d', ['help', 'my_file='])
print(opts)
print(args)
except getopt.GetoptError:
print('Something Wrong!')
sys.exit(2)

Result

>python CommandLineOption.py -m 2 -h "Help" H E L L O
[('-m', '2'), ('-h', '')]
['Help', 'H', 'E', 'L', 'L', 'O']

Python argparse module

Python argparse module is the preferred way to parse command line arguments. This module provides several options like positional arguments, default value for arguments, help message, specifying data type of argument etc.

Below is a simple program to understand argparse module -

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("string", help="Print the word in upper case letters")
args = parser.parse_args()
print(args.string.upper()) # This way argument can be manipulated.

Output

>python CommandLineOption.py "Hello, pythoN"
HELLO, PYTHON

We can set optional argument with argparse module like –verbosity.