Caesar Cypher Encoder/Decoder

caesar_cypher.py

'''Caesar Cypher encoder/decoder

From input, this program will generate Caesar cypher(each alphabets of input will be changed 3 step forward alphabets
in alphabet order. It will print the outcome.

Then, the program will decode the generated Caesar cypher and print it.

The user can compare her/his original input to the printed outcome.
'''


import string

#We will not change digits, whitespaces or punctuations. Those will remain the same.
LOWER = string.ascii_lowercase
SKIP = string.digits + string.whitespace + string.punctuation

#Function that encodes plain text to cypher. It changes any kinds of alphabet 3 stepts forward('A' will become 'D')
#About X, Y, Z, x, y, z, X will become 'A' and z will become 'c'.(uppercase alphabet loop for uppercase, lower for lower)
def caesar_cipher_encode(plain_text: str, step=3):
    cipher = ''

    for c in plain_text:
        if c in SKIP: #punctuations, whitespaces, digits
            cipher += c
            continue
        diff = ord(c) - (ord('a') if c in LOWER else ord('A'))
        #For last three characters in alphabet order:
        diff = (diff + step) % 26

        cipher += chr(diff + (ord('a') if c in LOWER else ord('A')))
    return cipher


#Get an input of a string from the user.
strInput = input("Please enter a string: ")

#Encode the input and print it
encode = caesar_cipher_encode(strInput)
print(encode)

#Decode the cypher(which is encoded input) and print it.
#What we have to do is just changing step=3 to step=-3
decode = caesar_cipher_encode(encode, step=-3)
print(decode)