Python Command‑Line Arguments

1. Why Use Command‑Line Arguments?

• Make scripts flexible without editing code.
• Automate tasks with shell scripts or build tools.
• Pass filenames, options, or configuration at runtime.

2. Quick & Easy: sys.argv

sys.argv is a list containing the script name and any arguments.

import sys

# python greet.py John 55
script, name, age = sys.argv
print(f"Hello, {name}! You are {age} years old.")

• sys.argv[0] is always the script name.
• All values are strings—convert types as needed.

3. Robust Parsing: argparse

The argparse module provides automatic help messages, type conversion, default values, and more.

import argparse

parser = argparse.ArgumentParser(description="Greet a user.")
parser.add_argument("name", help="Name of the user")
parser.add_argument("age", type=int, help="Age of the user")
parser.add_argument("-v", "--verbose", action="store_true",
                    help="Enable verbose output")

args = parser.parse_args()

greeting = f"Hello, {args.name}! You are {args.age}."
if args.verbose:
    greeting += " Nice to meet you!"

print(greeting)

Run python greet.py John 55 --verbose to see the optional flag in action.

4. Common argparse Features

• Types & Validation: type=int, choices=[...]
• Default Values: parser.add_argument("--level", default=1)
• Boolean Flags: action="store_true"
• Help Text: Automatically generated with -h/--help
• Mutually Exclusive Options: parser.add_mutually_exclusive_group()

5. Best Practices

• Use argparse for anything beyond trivial scripts.
• Provide clear help messages and sensible defaults.
• Validate inputs with type= or custom functions.
• Keep flag names short (-o) and long (--output) for usability.
• Document usage examples in your project README.