Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 07:18:10) [MSC v.1900 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> ALPHABET = 'abcdefghijklmnopqrstuvwxyz'

def Menu():
    print("Please choose from the following: \n")
    print("'e' to encode a string.")
    print("'d' to decode a string.")
    print("'q' to quit.\n")
    choice = input("Please enter one of the letters above.\n")
    if choice == "e":
        Encode()
    if choice == "d":
        Decode()
    if choice == "q":
        print("The program will now exit.")
        quit()

def stringValidation():
    while True:
        try:
            valid = str(input("Enter a string to encode.\n"))
            return valid
            break
        except:
            print("Value Error. Enter a string with only letters from the alphabet.")
            continue

def shiftValidation():
    while True:
        try:
            valid = int(input("Enter the number of shifts you would like.\n"))
            return valid
            break
        except:
            print("Value Error. Please enter an integer.")

def shiftValidation2():
    while True:
        try:
            valid = int(input("Enter the number of shifts you would like.\n"))
            return valid
            break
        except:
            print("Value Error. Please enter an integer.")

def Encode(shift):
    data = []
    string = stringValidation()
    shift = shiftValidation()
    for i in string:
        if i.strip() and i in ALPHABET:
            data.append(ALPHABET[(ALPHABET.index(i) + shift) % 26])
        else:
            data.append(i)
    output = ''.join(data)
    return output

def Decode():
    print("Hello.")


Menu()