Python programs can accept input directly from the command line using the argparse module, part of the standard library. This module provides a robust and flexible way to define and parse commend-line options, arguments, and subcommends.

The core workflow involves creating an ArgumentParser object, defining expected arguments with add_argument(), and then parsing the actual command-line input with parse_args(). Each defined argument maps to an attribute on the resulting namespace object.

Positional Arguments with Variable Count

Use nargs='*' to collect zero or more positional arguments into a list:

import argparse

parser = argparse.ArgumentParser(description='Process multiple files')
parser.add_argument('files', metavar='FILE', nargs='*')
args = parser.parse_args()
print(args.files)

Running this script:

python script.py a.txt b.txt c.txt

outputs: ['a.txt', 'b.txt', 'c.txt'].

Boolean Flags

For simple on/off flags, use action='store_true':

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('-v', '--verbose', action='store_true',
                    help='Enable verbose output')
args = parser.parse_args()
print(args.verbose)

If -v or --verbose is provided, args.verbose becomes True; otherwise, it defaults to False.

Single-Value Options

To accept a single value for an option (the default behavior), use action='store' (which is implicit):

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('-o', '--output', dest='output_file',
                    help='Specify output file path')
args = parser.parse_args()
print(args.output_file)

Collectnig Multiple Values

Use action='append' to accumulate multiple occurrences of an option into a list:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('-p', '--pattern', dest='patterns', action='append',
                    help='Add a search pattern')
args = parser.parse_args()
print(args.patterns)

Example usage:

python script.py -p "error" --pattern "warning"

Results in: ['error', 'warning'].

Restricted Choices and Defaults

Constrain acceptable values using the choices parameter and provide a fallback with default:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--mode', choices=['fast', 'slow'], default='slow',
                    help='Set processing speed')
args = parser.parse_args()
print(args.mode)

This ensures only 'fast' or 'slow' are accepted; if omitted, 'slow' is used.