Python script that encrypts a string using the Caesar cipher with argparse.

caesar_encrypt_argparse.py

#!/usr/bin/env python3
"""
caesar_encrypt.py

Encrypt a string with the Caesar cipher using argparse.

Usage:
    $ python caesar_encrypt.py "Hello, World!" --shift 5
    $ python caesar_encrypt.py "Attack at dawn!"
"""
import argparse


def caesar_shift_char(ch: str, shift: int) -> str:
    if ch.isupper():
        base = ord('A')
    elif ch.islower():
        base = ord('a')
    else:
        return ch
    offset = ord(ch) - base
    new_offset = (offset + shift) % 26
    return chr(base + new_offset)


def caesar_encrypt(text: str, shift: int) -> str:
    return ''.join(caesar_shift_char(ch, shift) for ch in text)


def main() -> None:
    parser = argparse.ArgumentParser(description="Caesar cipher encryptor")
    parser.add_argument("text", help="Text to encrypt")
    parser.add_argument(
        "--shift",
        type=int,
        default=3,
        help="Number of positions to shift (default: 3)",
    )
    args = parser.parse_args()

    ciphertext = caesar_encrypt(args.text, args.shift)
    print("\nCiphertext:\n")
    print(ciphertext)


if __name__ == "__main__":
    main()