My Caesar Code | Python Fiddle

code = {'A': 0, 'B': 1,'C': 2,'D': 3,
'E': 4,'F': 5,'G': 6,'H': 7,
'I': 8,'J': 9,'K': 10,'L': 11,
'M': 12,'N': 13,'O': 14,'P': 15,
'Q': 16,'R': 17,'S': 18,'T': 19,
'U': 20,'V': 21,'W': 22, 'X': 23,
'Y': 24,'Z': 25}

reverse_code = dict((v,k) for k, v in code.iteritems()) #specify reverse-
#associated dictionary

def encoder(message, key_word):
    
    list_of_message = []
    key = []
    encoded_message = []
    
    for a in message:#convert string of letters to list of associated numbers
        if a in code:
            list_of_message.append(code[a])
        else:
            list_of_message.append(a)

while len(key) < len(list_of_message):#create the key to encode the message
        for a in key_word:
            key.append(code[a])
    key = key[0:len(list_of_message)]#set length of key equal to length of messsage
    
    for a,b in zip(key, list_of_message):#create encoded message
        if b not in range(0,26):
             encoded_message.append(b)#transfer non-encoded elements
        else: #encolde elements
            c = b + a 
            if c > 25:#this is quivalent to looping the dictionary.
                c = c - 25
            encoded_message.append(reverse_code[c])#use reverse dictionary to
            #convert back into letters 
            
    encoded_message = ''.join(encoded_message)#convert encoded message to string
    print "encoded message:", encoded_message
    return encoded_message
    
def decoder(encoded_message, key_word):

decoded_list = []
    key = []
    message = []
    
    for a in encoded_message:
        if a in code:
            decoded_list.append(code[a])
        else:
            decoded_list.append(a)
    
    while len(key) < len(decoded_list):
        for a in key_word:
            key.append(code[a])
    key = key[0:len(decoded_list)]
    
    for a,b in zip(key, decoded_list):
        if b not in range(0,26):
            message.append(b)
        else:
            c = b - a 
            if c < 0:
                c += 25
            message.append(reverse_code[c])
    message = ''.join(message)
    print 'decoded message:', message
    return message

def encode_or_decode():
    
    encode_decode = raw_input("do you want to encode or decode? ")  #.ascii_lowercase

message = raw_input("Enter message: ")#.ascii_uppercase
    key_word = raw_input("Enter key: ")#.ascii_uppercase

if encode_decode == "encode":
        encoder(message, key_word)
    if encode_decode == "decode":
        decoder(message, key_word)

encode_or_decode()

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