Python：itertools模块 combinations和product的使用

1.combinations(iterable, r)  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:官方文档　　

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)

例：

>>> list(combinations(range(3),2))
[(0, 1), (0, 2), (1, 2)]
>>> list(combinations(range(3),3))
[(0, 1, 2)]
>>> list(combinations(range(3),1))
[(0,), (1,), (2,)]

2.product(*iterables[, repeat])  创建一个迭代器，生成表示iterables中的项目的笛卡尔积的元组，repeat表示重复生成序列的次数。

def product(*args, **kwds):
# product('ABCD', 'xy') --> Ax Ay Bx By Cx Cy Dx Dy
# product(range(2), repeat=3) --> 000 001 010 011 100 101 110 111
pools = map(tuple, args) * kwds.get('repeat', 1)
result = [[]]
for pool in pools:
result = [x+[y] for x in result for y in pool]
for prod in result:
yield tuple(prod)

 例:

>>> list(product(range(3),repeat=1))
[(0,), (1,), (2,)]
>>> list(product(range(3),repeat=2))
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]
>>> list(product(range(3),repeat=3))
[(0, 0, 0), (0, 0, 1), (0, 0, 2), (0, 1, 0), (0, 1, 1), (0, 1, 2), (0, 2, 0), (0, 2, 1), (0, 2, 2), (1, 0, 0), (1, 0, 1), (1, 0, 2), (1, 1, 0), (1, 1, 1), (1, 1, 2), (1, 2, 0), (1, 2, 1), (1, 2, 2), (2, 0, 0), (2, 0, 1), (2, 0, 2), (2, 1, 0), (2, 1, 1), (2, 1, 2), (2, 2, 0), (2, 2, 1), (2, 2, 2)]

转载于：https://www.cnblogs.com/yu-zhang/p/3556232.html