Python - Cuadrado de Polibio

#-*- coding:utf-8 -*-

lookup_table = {'A' : '11', 'B' : '12', 'C' : '13', 'D' : '14', 'E' : '15',

'F' : '21', 'G' : '22', 'H' : '23', 'I' : '24', 'J' : '24',

'K' : '25', 'L' : '31', 'M' : '32', 'N' : '33', 'O' : '34',

'P' : '35', 'Q' : '41', 'R' : '42', 'S' : '43', 't' : '44',

'U' : '45', 'V' : '51', 'W' : '52', 'X' : '53', 'Y' : '54', 'Z' : '55'}

def atbash(message):

    cadena="".join(message.split())

    cipher = ''

    if cadena.isalpha():

        for letter in message.upper():

            if letter != ' ':

                cipher += lookup_table[letter]

            else:

                cipher += ' '

    else:

        list_of_key = list(lookup_table.keys())

        list_of_value = list(lookup_table.values())

        texto = "00".join(message.split())

        sep = []

        for i in range(0,len(texto),2):

            sep.append(texto[i:i+2])

        for x in sep:

            if x != "00":

                position = list_of_value.index(x)

                cipher+= "".join(list_of_key[position])

            else:

                cipher+= " "

    return cipher

def main():

    message1 = 'deseamos la paz'

    message2 = '1415431511323443 3111 351155'

    print(atbash(message1))

    print(atbash(message2))

if __name__ == '__main__':

    main()