ROT13 encoder/decoder

rot13.py

#!/usr/bin/python

from string import lowercase, uppercase

ROTATED_LOWERCASE = lowercase[13 : ] + lowercase[ : 13]
ROTATED_UPPERCASE = uppercase[13 : ] + uppercase[ : 13]
REF = {}

for index1 in range(len(lowercase)):
    REF[lowercase[index1]] = ROTATED_LOWERCASE[index1]

for index2 in range(len(uppercase)):
    REF[uppercase[index2]] = ROTATED_UPPERCASE[index2]

def string_rotation(input_str):
    '''
    A function that takes a white-space separated string and returns
    a rot13 version of that string with every other word reversed.
    If the function is f, f(f(x)) returns x. 
    '''
    if type(input_str) == str:
        fin = []
        input_list = input_str.split()
        for index in range(len(input_list)):
            temp = ''
            if index % 2 == 0:
                for char in input_list[index][::-1]:
                    if char in REF:
                        temp += REF[char]
                    else:
                        temp += char
            else:
                for char in input_list[index]:
                    if char in REF:
                        temp += REF[char]
                    else:
                        temp += char
            fin.append(temp)
        return ' '.join(fin)
    else:
        return 'The argument has to be a string!'
    

if __name__ == '__main__':
    OUTPUT = string_rotation("Hello world! This is ROT13 :)")
    print OUTPUT
    print string_rotation(OUTPUT)