Python中的itertools.combinations(iterable, r)（关键词：itertools/combinations）

创建一个迭代器，返回iterable中所有长度为r的子序列，返回的子序列中的项按输入iterable中的顺序排序 (不带重复)

def combinations(iterable, r):
     # combinations('ABCD', 2) --> AB AC AD BC BD CD
     # combinations(range(4), 3) --> 012 013 023 123
     pool = tuple(iterable)
     n = len(pool)
     if r > n:
         return
     indices = range(r)
     yield tuple(pool[i] for i in indices)
     while True:
         for i in reversed(range(r)):
             if indices[i] != i + n - r:
                 break
         else:
             return
         indices[i] += 1
         for j in range(i+1, r):
             indices[j] = indices[j-1] + 1
         yield tuple(pool[i] for i in indices)

#或者
def combinations(iterable, r):
     pool = tuple(iterable)
     n = len(pool)
     for indices in permutations(range(n), r):
         if sorted(indices) == list(indices):
             yield tuple(pool[i] for i in indices)

参考文献：
1. Python itertools模块详解 - 脚本之家；
2. itertools - Python标准库。