calling upper() on a string, not working?

[John Salerno]

Can someone tell me what's happening here. This is my code:



PUNCT_SPACE_SET = set(string.punctuation + string.whitespace)

def filter_letters(original):
     return ''.join(set(original) - PUNCT_SPACE_SET)



'original' is a string. The above works as expected, but when I change 
it to

return ''.join(set(original.upper()) - PUNCT_SPACE_SET)

it doesn't seem to work. The full code is below if it helps to understand.



import string
import random
import itertools

PUNCT_SPACE_SET = set(string.punctuation + string.whitespace)

def convert_quote(quote):
     return encrypt_quote(quote).split('|')

def encrypt_quote(original):
     original_letters = filter_letters(original)
     new_letters = list(string.ascii_uppercase)
     while True:
         random.shuffle(new_letters)
         trans_letters = ''.join(new_letters)[:len(original_letters)]
         if test_code(original_letters, trans_letters):
             trans_table = string.maketrans(original_letters, trans_letters)
             break
     return original.translate(trans_table)

def filter_letters(original):
     return ''.join(set(original) - PUNCT_SPACE_SET)
     #return ''.join(set(original.upper()) - PUNCT_SPACE_SET)

def test_code(original_letters, trans_letters):
     for pair in itertools.izip(original_letters, trans_letters):
         if pair[0] == pair[1]:
             return False
     return True

if __name__ == '__main__':
     print convert_quote("The past is not dead. In fact, it's not even 
past.|William Faulkner")