caesar.py

import cs50
import sys

if len(sys.argv) == 1 or len(sys.argv) > 2:
    print("Usage: python caesar.py <key>")
    exit(1)

print("plaintext: ", end="")
plaintext = cs50.get_string()
key = int(sys.argv[1])
size = len(plaintext)
ciphertext = []

if key >= pow(2, 31) - 26 or key <= 0:
    print("The value for the key is invalid!")
else:
    if key > 26:
        key %= 26
    for i in range(size):
        if plaintext[i].isalpha():
            c = (ord(plaintext[i]) + key)
            if plaintext[i].upper() > 'Z':
                c -= 26
        ciphertext.append(chr(c))

print("ciphertext: ", end="")
print("".join(ciphertext))
exit(0)