使用递归生成组合数.

combinations.py

def combinations(iterable, r):
    pool = tuple(iterable)
    n = len(pool)
    if r > n:
        return

    def comb_core(n, r):
        for i in reversed(range(r - 1, n)):
            if (r == 1):
                yield (i, )
            else:
                for out in comb_core(i, r - 1):
                    yield (i, ) + out

    for out in comb_core(n, r):
        yield tuple(pool[i] for i in out)