0 def combinations(iterable, r): 1     # combinations('ABCD', 2) --> AB AC AD BC BD CD 2     # combinations(range(4), 3) --> 012 013 023 123 3     pool = tuple(iterable) 4     n = len(pool) 5     if r > n: 6         return 7     indices = range(r) 8     yield tuple(pool[i] for i in indices) 9     while True:10         for i in reversed(range(r)):11             if indices[i] != i + n - r:12                 break13         else:14             return15         indices[i] += 116         for j in range(i+1, r):17             indices[j] = indices[j-1] + 118         yield tuple(pool[i] for i in indices)