argparse Module Complexity

The argparse module provides a framework for creating command-line argument parsers with automatic help and usage messages.

Complexity Reference

Operation	Time	Space	Notes
ArgumentParser()	O(1)	O(1)	Create parser
add_argument()	O(1)	O(1)	Add single argument
parse_args()	O(n)	O(n)	n = number of CLI args
Help generation	O(m)	O(m)	m = total help text

Basic Usage

Creating a Parser

import argparse

# Create parser - O(1)
parser = argparse.ArgumentParser(description='My program')  # O(1)

# Add arguments - O(1) each
parser.add_argument('name', help='Your name')  # O(1)
parser.add_argument('--age', type=int, help='Your age')  # O(1)

# Parse arguments - O(n)
args = parser.parse_args()  # O(n) where n = CLI args
print(f"Name: {args.name}, Age: {args.age}")

Argument Types

import argparse

parser = argparse.ArgumentParser()

# Positional arguments - O(1)
parser.add_argument('input_file', help='Input file path')

# Optional arguments - O(1)
parser.add_argument('-o', '--output', help='Output file')
parser.add_argument('-v', '--verbose', action='store_true')  # Boolean flag
parser.add_argument('-n', '--number', type=int, default=10)  # Integer

# Parse - O(n)
args = parser.parse_args()

Advanced Features

Choices and Validation

import argparse

parser = argparse.ArgumentParser()

# Limited choices - O(1)
parser.add_argument('--format', choices=['json', 'csv', 'xml'])

# Type conversion - O(1)
parser.add_argument('--count', type=int)

# Custom validation
def positive_int(value):
    ivalue = int(value)  # O(k) where k = string length
    if ivalue <= 0:
        raise argparse.ArgumentTypeError(f"{value} must be positive")
    return ivalue

parser.add_argument('--positive', type=positive_int)

Subcommands

import argparse

# Create main parser - O(1)
parser = argparse.ArgumentParser()

# Create subparsers - O(1)
subparsers = parser.add_subparsers(dest='command')

# Add subcommands - O(1) each
parser_a = subparsers.add_parser('add', help='Add item')
parser_a.add_argument('item')

parser_r = subparsers.add_parser('remove', help='Remove item')
parser_r.add_argument('item')

# Parse - O(n)
args = parser.parse_args()

Common Patterns

Command-Line Tool

import argparse

def main():
    # Create parser - O(1)
    parser = argparse.ArgumentParser(
        description='File processor',
        epilog='Example: %(prog)s input.txt -o output.txt'
    )

    # Add arguments - O(1) each
    parser.add_argument('input', help='Input file')
    parser.add_argument('-o', '--output', help='Output file')
    parser.add_argument('-v', '--verbose', action='count', default=0)

    # Parse - O(n)
    args = parser.parse_args()

    # Process
    process(args.input, args.output, args.verbose)

if __name__ == '__main__':
    main()

Performance Notes

Parsing Speed

import argparse
import time

parser = argparse.ArgumentParser()
parser.add_argument('--verbose', action='store_true')
parser.add_argument('--count', type=int)

# Parsing is O(n) where n = number of arguments
args_list = ['--verbose', '--count', '10']

start = time.time()
args = parser.parse_args(args_list)  # O(3)
elapsed = time.time() - start
# Very fast for typical argument counts

Version Notes

• Python 2.x: argparse available as backport
• Python 3.x: Built-in since Python 3.2
• All versions: O(n) parsing complexity

Related Modules

• sys.argv - Raw command-line arguments
• click - Alternative argument parsing (external)

Best Practices

✅ Do:

• Use argparse for command-line tools
• Provide helpful descriptions
• Use subparsers for complex tools
• Validate arguments with type and choices

❌ Avoid:

• Manual sys.argv parsing
• Unclear help messages
• Too many positional arguments