Python Caesar Cipher Encryption & Decryption

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#==============MODULES================#
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#=====================================#
#==============VARIABLES==============#
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alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
alpha_size = len(alpha)
menus = ["(1) Encrypt", "(2) Decrypt", "(3) Exit   "]
max_set = ord("Z")
min_set = ord("A")

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#==============FUNCTIONS==============#
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def menu(parameter):
    print "     +===========+"
    print "     |   MENU    |"
    print "     |-----------|"
    for word in parameter:
        print "     |%s|" % word
        print "     |-----------|"

#define encryption/decryption algorithm
def algo(data, key):
    output = ""
    for char in data: #iterates through each character in string
        if char.isalpha(): #if character is True....
            nums = ord(char) #convert character to ascii number
            nums = (nums + key) #shift ascii number with key
            if nums > max_set: #if new ascii character higher than ascii "Z"
                nums = (nums - alpha_size) #subtract 26 to bring character in Modulo 26
            elif nums < min_set: #if new ascii character lower than ascii "A"
                nums = (nums + alpha_size) #add 26 to bring the character in Modulo 26
            output = (output + chr(nums)) #append blank string with converted ascii to new character
        else:
            output = (output + char) #append string with special characters and numbers b/c of established modulo
    print output

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#===========MAIN PROGRAM==============#
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def main():
    menu(menus)

    choice = input("Please make a selection(1-3): ")

    #Encryption
    if choice == 1:
        key = input("Please enter a key(0-25): ")
        data = raw_input("Please enter a message to encrypt: ").upper()
        print "The encrypted message is: "
        algo(data, key)

    #Decryption
    elif choice == 2:
        key = input("Please enter a key(0-25): ")
        key = (key * -1)
        data = raw_input("Please enter a message to decrypt: ").upper()
        print "The decrypted message is: ")
        algo(data, key)

    #Exit
    elif choice == 3:
        print "Exiting Program. Goodbye"
        return

    #Fault
    else:
        print "Invalid Selection"
        return main()