# do this once, outside the loop
alphabet=set(string.ascii_lowercase)
# inside the loop, just 1 line:
missingletter=(alphabet-set(yourlist)).pop()

frompos, topos = 0, len(str)
for i in range(1,100):  #never say forever with bisection...
    trypos = (frompos+topos+1)/2
    print "try:",frompos,trypos,topos
    if alphabet[trypos] != str[trypos]:
        topos = trypos
    else:
        frompos = trypos
    if topos-frompos==1:
        if alphabet[topos] != str[topos]:
            print alphabet[frompos]
        else:
            print alphabet[topos]
        break

>>> import string
>>> sourcelist = 'abcdefghijklmnopqrstuvwx'
>>> [letter for letter in string.ascii_lowercase if letter not in sourcelist]
['y', 'z']
>>>

>>> string.ascii_lowercase
'abcdefghijklmnopqrstuvwxyz'

>>> string.letters
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'

>>> string.hexdigits
'0123456789abcdefABCDEF'

>>> string.octdigits
'01234567'

>>> string.digits
'0123456789'

>>> string.printable
'0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~ tnrx0bx0c'
>>>

print chr(2847 - sum(map(ord, theString)))

def missing_letter_basic(s):
    for letter in string.ascii_lowercase:
        if letter not in s: return letter
    raise Exception("No missing letter")

find_missing_letter = dict((string.ascii_lowercase[:i]+string.ascii_lowercase[i+1:],
                            string.ascii_lowercase[i]) for i in range(26)).get

>>> find_missing_letter('abcdefghijklmnoprstuvwxyz')
'q'

"b"    "m"     "y"
               bisect : 2.762   2.872   2.922  (Phil H)
             find_gap : 3.388   4.533   5.642  (unwind)
             listcomp : 2.832   2.858   2.822  (monkut)
         listcomp_set : 4.770   4.746   4.700  As above, with sourcelist=set(sourcelist) first
       set_difference : 2.924   2.945   2.880  (Phil H)
                  sum : 3.815   3.806   3.868
             sum_imap : 3.284   3.280   3.260
                basic : 0.544   1.379   2.359
          dict_lookup : 0.135   0.133   0.134

for i in xrange(1, len(a)):
  if a[i] != a[i - 1] + 1:
    print a[i - 1] + 1, "is missing"

letterSet = set()
for word in wordList:
  letterSet.update(set(word.lower()))

import string
alphabet = set(string.lowercase)
missingLetters = alphabet.difference(letterSet)

class MissingFinder(object):
    "A simplified missing items locator"
    def __init__(self, alphabet):
        "Store a set from our alphabet"
        self.alphabet= set(alphabet)
    def missing(self, sequence):
        "Return set of missing letters; sequence not necessarily set"
        return self.alphabet.difference(sequence)

>>> import string
>>> finder= MissingFinder(string.ascii_lowercase)

>>> finder.missing(string.ascii_lowercase[:5] + string.ascii_lowercase[6:])
>>> set(['f'])
>>> # rinse, repeat calling finder.missing