simple code of Caesar Cipher in python

CaesarCipher.py

arr = "abcdefghijklmnopqrstuwxyz"

key = 3
plain = "hello world 2day"
cyper = "";
for c in plain:
    if c.isalpha() : cyper += arr[(arr.index(c)+3)%26]
    else: cyper += c

print(cyper)