python怎么生成list的所有元素的组合

Question

比如l = [1,2,3]
指定长度2，就会得到[1,2][2,3][1,3]，不考虑顺序。

Answer 1

生成排列可以用product：

from itertools import product
l = [1, 2, 3]
print list(product(l, l))
print list(product(l, repeat=4))

组合的话可以用combinations：

from itertools import combinations
print list(combinations([1,2,3,4,5], 3))

下面是我以为没有combinations然后自己写的，没有itertools的python（2.6以下）可供参考。

import copy

def combine(l, n): 
    answers = []
    one = [0] * n 
    def next_c(li = 0, ni = 0): 
        if ni == n:
            answers.append(copy.copy(one))
            return
        for lj in xrange(li, len(l)):
            one[ni] = l[lj]
            next_c(lj + 1, ni + 1)
    next_c()
    return answers

print combine([1, 2, 3, 4, 5], 3)

输出：

[[1, 2, 3], [1, 2, 4], [1, 2, 5], [1, 3, 4], [1, 3, 5], [1, 4, 5], [2, 3, 4], [2, 3, 5], [2, 4, 5], [3, 4, 5]]

Answer 2

import itertools
nums = itertools.permutations([1,2,3])
for x in  nums:
    print(x)

>>> 
(1, 2, 3)
(1, 3, 2)
(2, 1, 3)
(2, 3, 1)
(3, 1, 2)
(3, 2, 1)