Command Line Arguments¶

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

Mental Model

argparse turns your script into a proper command-line tool. You declare what arguments and flags your program accepts, and argparse parses sys.argv, validates inputs, generates help text, and returns a namespace of clean values. It is the standard-library alternative to manually slicing sys.argv.

Basic Usage¶

```python 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: ```bash python script.py 5 3

Sum: 8.0¶

```

Positional Arguments¶

Positional arguments are required and parsed in order.

```python 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}") ```

Keyword (Optional) Arguments¶

Use - or -- prefix for optional arguments.

```python 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() ```

The args Namespace¶

parse_args() returns a Namespace object:

```python 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) # '+' ```

Use args.num1, args.num2, and args.op to access values safely.

Exercises¶

Exercise 1. Write a script that uses argparse to accept a filename and an optional --verbose flag. When --verbose is set, print additional information about the file (size, modification time).

Solution to Exercise 1

import argparse
import os
import time

parser = argparse.ArgumentParser(description="Display file info")
parser.add_argument("filename", help="Path to the file")
parser.add_argument("--verbose", "-v", action="store_true",
                    help="Show detailed file information")
args = parser.parse_args()

if not os.path.exists(args.filename):
    print(f"Error: {args.filename} not found")
else:
    print(f"File: {args.filename}")
    if args.verbose:
        stat = os.stat(args.filename)
        print(f"  Size: {stat.st_size} bytes")
        print(f"  Modified: {time.ctime(stat.st_mtime)}")

Exercise 2. Extend the script from Exercise 1 to support subcommands: info (display file information) and count (count lines, words, and characters in a text file).

Solution to Exercise 2

import argparse
import os
import time

parser = argparse.ArgumentParser(description="File utility")
subparsers = parser.add_subparsers(dest="command", required=True)

info_parser = subparsers.add_parser("info", help="Show file info")
info_parser.add_argument("filename")

count_parser = subparsers.add_parser("count", help="Count lines/words/chars")
count_parser.add_argument("filename")

args = parser.parse_args()

if args.command == "info":
    stat = os.stat(args.filename)
    print(f"Size: {stat.st_size} bytes")
    print(f"Modified: {time.ctime(stat.st_mtime)}")
elif args.command == "count":
    with open(args.filename) as f:
        content = f.read()
    lines = content.count("\n")
    words = len(content.split())
    chars = len(content)
    print(f"Lines: {lines}, Words: {words}, Characters: {chars}")