A-Z iterator

def sss( n ):
    t = [ 0 ] * n
    while True:
        p, tn = 1, []
        for x in t:
            tn.append( ( x + p ) % 26 )
            p = ( x + p ) / 26
        t = tn

        if p:
            raise StopIteration

        yield ''.join(reversed([ chr( x + 0x61 ) for x in t ]))


print len( list(sss( 4 ) ))