def all_binary_combinations(mycollection):
    for i in xrange(len(mycollection)):
        for combination in itertools.combination(mycollection, i):
            yield combination

def force_decid(userid_str):
    lower_inds = [i for i, char in enumerate(userid_str) if char.islower()]
    for combination in all_binary_combinations(lower_inds):
        cp = ''.join(char.upper() if i in lower_inds else char for i, char in enumerate(cp))
        try decid(cp):
            return cp
        expecpt InvalidID:
            pass