Secret code: the Caesar cipher in Python.

The program

Press Run to see what it prints. Change anything you like, then run it again.

def shift(text, key):
    result = ""
    for ch in text:
        if ch.isalpha():
            base = ord("a") if ch.islower() else ord("A")
            result = result + chr((ord(ch) - base + key) % 26 + base)
        else:
            result = result + ch
    return result

secret = shift("Meet me at noon!", 3)
print(secret)
print(shift(secret, -3))

Phhw ph dw qrrq!
Meet me at noon!

How it works

1. Julius Caesar is said to have used this code, moving each letter three places. A becomes D, B becomes E and so on.
2. ord turns a letter into its number and chr turns a number back into a letter. % 26 wraps round from Z to A.
3. To decode, shift the other way: shift(secret, -3).

Try this

• Try a different key.
• Encrypt your name.
• Try every key from 1 to 25 on a secret message and look for the one that makes sense.

Stuck on an error? Read how to debug Python.