codice:
#!usr/bin/python
# -*- coding: utf-8 -*-

import os

non_validi  = ""
non_validi += "0123456789"
non_validi += """àèìòù@#]\|!"£$%&/()=?^'*°§ç.:-_ """

fi1 = open('fi1.txt','r')
fi2 = open('fi2.txt','r')
fo = open('fo.txt','w')
lista1 = ''
lista2 = ''
diff = ''

for line in fi1.readlines():
    for i in line:
        if (i not in non_validi):
            lista1+= i

for line in fi2.readlines():
    for i in line:
        if (i not in non_validi):
            lista2+= i
            
lista1 = lista1[:-1].split(',')
lista2 = lista2[:-1].split(',')

print lista1
print lista2

for i in lista1:
    if i not in lista2:
        diff += i+','

for i in lista2:
    if i not in lista1:
        diff += i+','

fo.write(diff[:-1])
fi1.close()
fi2.close()
fo.close()