Understanding Caesar cipher in Python — encode and decode text easily

SOLVED: A Caesar cipher is a simple substitution cipher based on the idea of shifting each letter of the plaintext message a fixed number (called the key) of positions in the alphabet. For example, if the key value is 2, the word "Sourpuss" would be encoded as "Uqwtrwuu." The original message can be recovered by "reencoding" it using the negative of the key.Write a program that can encode and decode Caesar ciphers. The input to the program will be a string of plaintext and the value of the key.