Command Line Arguments

The argparse module provides a way to parse command-line arguments passed to Python scripts.

Basic Usage

import argparse

parser = argparse.ArgumentParser(description='A simple calculator')
parser.add_argument("num1", type=float, help="First number")
parser.add_argument("num2", type=float, help="Second number")

args = parser.parse_args()
print(f"Sum: {args.num1 + args.num2}")

Run:

python script.py 5 3
# Sum: 8.0

Positional Arguments

Positional arguments are required and parsed in order.

import argparse

def calculate(x, y, op):
    if op == "+": return x + y
    if op == "-": return x - y
    if op == "*": return x * y
    if op == "/": return x / y

parser = argparse.ArgumentParser(description='Calculator')
parser.add_argument("num1", type=float, help="First number")
parser.add_argument("num2", type=float, help="Second number")
parser.add_argument("op", help="Operation: +, -, *, /")

args = parser.parse_args()
result = calculate(args.num1, args.num2, args.op)
print(f"Result: {result}")

Run:

python script.py 10 5 +
# Result: 15.0

Keyword (Optional) Arguments

Use - or -- prefix for optional arguments.

import argparse

parser = argparse.ArgumentParser(description='Calculator')
parser.add_argument("-x", "--num1", type=float, default=0, help="First number")
parser.add_argument("-y", "--num2", type=float, default=0, help="Second number")
parser.add_argument("-o", "--op", default="+", help="Operation")

args = parser.parse_args()

Run:

python script.py                    # Uses defaults
python script.py -x 10 -y 5         # num1=10, num2=5, op="+"
python script.py --num1 10 --num2 5 # Same as above
python script.py -x 10 -y 5 -o "*"  # Multiplication

Default Values

parser.add_argument("-n", "--name", default="World", help="Name to greet")

If not provided, args.name will be "World".

Type Conversion

By default, arguments are strings. Use type= to convert:

parser.add_argument("count", type=int)
parser.add_argument("--rate", type=float, default=0.1)

Boolean Flags

Use action="store_true" for boolean flags:

parser.add_argument("-v", "--verbose", action="store_true", help="Verbose output")

Run:

python script.py           # args.verbose = False
python script.py -v        # args.verbose = True
python script.py --verbose # args.verbose = True

Help Text

argparse automatically generates help:

python script.py --help

Output:

usage: script.py [-h] [-x NUM1] [-y NUM2] [-o OP]

Calculator

optional arguments:
  -h, --help            show this help message and exit
  -x NUM1, --num1 NUM1  First number
  -y NUM2, --num2 NUM2  Second number
  -o OP, --op OP        Operation

Complete Example

import argparse

def calculate(x, y, op):
    operations = {
        "+": x + y,
        "-": x - y,
        "*": x * y,
        "/": x / y if y != 0 else None
    }
    return operations.get(op)

def main():
    parser = argparse.ArgumentParser(description='Simple calculator')

    # Optional arguments with defaults
    parser.add_argument("-x", "--num1", type=float, default=1.0,
                        help="First number (default: 1.0)")
    parser.add_argument("-y", "--num2", type=float, default=2.0,
                        help="Second number (default: 2.0)")
    parser.add_argument("-o", "--op", default="+",
                        help="Operation: +, -, *, / (default: +)")

    args = parser.parse_args()

    result = calculate(args.num1, args.num2, args.op)
    if result is not None:
        print(f"{args.num1} {args.op} {args.num2} = {result}")
    else:
        print("Error: Division by zero")

if __name__ == "__main__":
    main()

The args Namespace

parse_args() returns a Namespace object:

args = parser.parse_args()
print(args)  # Namespace(num1=10.0, num2=5.0, op='+')

# Access as attributes
print(args.num1)  # 10.0
print(args.op)    # '+'

Common Patterns

Required Optional Argument

parser.add_argument("-f", "--file", required=True, help="Input file")

Choices

parser.add_argument("-o", "--op", choices=["+", "-", "*", "/"],
                    default="+", help="Operation")

Multiple Values

parser.add_argument("files", nargs="+", help="Input files")
# Usage: python script.py file1.txt file2.txt file3.txt

Summary

Feature	Syntax
Positional	parser.add_argument("name")
Optional	parser.add_argument("-n", "--name")
Type	type=int, type=float
Default	default=value
Required	required=True
Boolean	action="store_true"
Choices	choices=["a", "b", "c"]
Help	help="Description"