获取长度为 n 的所有（n-选择-k）组合

我怎样才能获得长度的所有组合（按顺序）n从数字列表中？例如，给定列表[1, 2, 3, 4]，并设置n = 3，我怎样才能得到这些结果？

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

_{For combinations of all possible lengths, see Get all possible (2^N) combinations of a list’s elements, of any length https://stackoverflow.com/questions/464864 . Note that this is not simply a matter of iterating over the possible lengths and combining the results, as there are other reasonable approaches to the problem.}

_{To avoid duplicate outputs when the input has duplicate elements, see Python combinations without repetitions https://stackoverflow.com/questions/36429507 .}

_{Also related: Generate all binary strings of length n with k bits set https://stackoverflow.com/questions/1851134}

itertools https://docs.python.org/3/library/itertools.html#itertools.combinations可以这样做：

import itertools

for comb in itertools.combinations([1, 2, 3, 4], 3):
    print(comb)

Outputs:

(1, 2, 3)
(1, 2, 4)
(1, 3, 4)
(2, 3, 4)