如何從k個項目列表中生成所有n大小的集合？

Question

給定一組k項（例如，{1,2,3}，對於k=3），如何使用原始組的值生成所有可能的長度爲n的組？應該有k**n這樣的列表。

Answer 1

可以使用itertools模塊：

>>> from itertools import permutations 
>>> s = set((1,2,3)) 
>>> list(permutations(s)) 

[(1, 2, 3), (1, 3, 2), (2, 1, 3), (2, 3, 1), (3, 1, 2), (3, 2, 1)] 

>>> list(permutations(s, 2)) 
[(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)] 

UPDATE： 也許這更像是在問：

>>> from itertools import chain, combinations_with_replacement, permutations 
>>> list(set(chain.from_iterable(permutations(x) for x in combinations_with_replacement(s, 3)))) 
[(1, 3, 2), 
(1, 3, 1), 
(3, 3, 1), 
(1, 1, 1), 
(3, 3, 3), 
(2, 3, 2), 
(3, 3, 2), 
(2, 3, 3), 
(2, 3, 1), 
(3, 2, 2), 
(3, 1, 3), 
(3, 2, 3), 
(3, 1, 2), 
(1, 2, 1), 
(3, 1, 1), 
(3, 2, 1), 
(1, 2, 2), 
(1, 2, 3), 
(2, 1, 2), 
(2, 2, 3), 
(2, 1, 3), 
(2, 2, 2), 
(1, 1, 3), 
(2, 1, 1), 
(1, 1, 2), 
(2, 2, 1), 
(1, 3, 3)]