Python: Argparse Command-Line Parsing

Table of Contents

• Basic Argparse Usage
• Common Argument Types
• Subcommands Example
• Mutually Exclusive Arguments
• Custom Validation

Basic Argparse Usage

#!/usr/bin/env python
import argparse
import logging

logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(__name__)

def parse_args():
    parser = argparse.ArgumentParser(
        description="Example script demonstrating argparse features"
    )

    # Required argument with custom destination
    parser.add_argument(
        "--arg",
        dest="arg_weird_dest",
        required=True,
        help="A required argument with custom destination name"
    )

    # Boolean flag (defaults to False)
    parser.add_argument(
        "--flg",
        action="store_true",
        help="Enable a feature (flag is False by default)"
    )

    return parser.parse_args()

if __name__ == "__main__":
    args = parse_args()
    logger.info(f"Argument value: {args.arg_weird_dest}")
    logger.info(f"Flag enabled: {args.flg}")

Common Argument Types

import argparse

parser = argparse.ArgumentParser()

# Positional argument (required by default)
parser.add_argument("input_file", help="Path to input file")

# Optional argument with default value
parser.add_argument("--output", default="output.txt", help="Output file path")

# Integer argument
parser.add_argument("--count", type=int, default=10, help="Number of iterations")

# Float argument
parser.add_argument("--threshold", type=float, default=0.5, help="Threshold value")

# Multiple values
parser.add_argument("--files", nargs="+", help="One or more file paths")

# Choice from a list
parser.add_argument(
    "--format",
    choices=["json", "yaml", "xml"],
    default="json",
    help="Output format"
)

# Store constant value
parser.add_argument(
    "--verbose", "-v",
    action="store_const",
    const=logging.DEBUG,
    default=logging.INFO,
    help="Enable verbose logging"
)

args = parser.parse_args()

Subcommands Example

import argparse

def cmd_create(args):
    print(f"Creating resource: {args.name}")
    if args.force:
        print("Force mode enabled")

def cmd_delete(args):
    print(f"Deleting resource: {args.name}")
    if args.confirm:
        print("Confirmed deletion")

parser = argparse.ArgumentParser(description="Resource management CLI")
subparsers = parser.add_subparsers(dest="command", help="Available commands")

# Create subcommand
create_parser = subparsers.add_parser("create", help="Create a new resource")
create_parser.add_argument("name", help="Resource name")
create_parser.add_argument("--force", action="store_true", help="Force creation")
create_parser.set_defaults(func=cmd_create)

# Delete subcommand
delete_parser = subparsers.add_parser("delete", help="Delete a resource")
delete_parser.add_argument("name", help="Resource name")
delete_parser.add_argument("--confirm", action="store_true", help="Confirm deletion")
delete_parser.set_defaults(func=cmd_delete)

args = parser.parse_args()
if hasattr(args, "func"):
    args.func(args)
else:
    parser.print_help()

Mutually Exclusive Arguments

import argparse

parser = argparse.ArgumentParser()

# Only one of these can be specified
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument("--read", action="store_true", help="Read mode")
group.add_argument("--write", action="store_true", help="Write mode")
group.add_argument("--delete", action="store_true", help="Delete mode")

args = parser.parse_args()

Custom Validation

import argparse
import os

def valid_file(path):
    """Custom validator for file paths"""
    if not os.path.isfile(path):
        raise argparse.ArgumentTypeError(f"{path} is not a valid file")
    return path

def valid_port(value):
    """Custom validator for port numbers"""
    ivalue = int(value)
    if ivalue < 1 or ivalue > 65535:
        raise argparse.ArgumentTypeError(f"{value} is not a valid port (1-65535)")
    return ivalue

parser = argparse.ArgumentParser()
parser.add_argument("--config", type=valid_file, help="Configuration file path")
parser.add_argument("--port", type=valid_port, default=8080, help="Server port")

args = parser.parse_args()