如何在Python中实现排列和组合的公式？

import itertools

# 从列表中选取3个元素进行排列
items = [1, 2, 3, 4]
permutations = list(itertools.permutations(items, 3))
print(permutations)

[(1, 2, 3), (1, 2, 4), (1, 3, 2), (1, 3, 4), (1, 4, 2), (1, 4, 3), (2, 1, 3), (2, 1, 4), (2, 3, 1), (2, 3, 4), (2, 4, 1), (2, 4, 3), (3, 1, 2), (3, 1, 4), (3, 2, 1), (3, 2, 4), (3, 4, 1), (3, 4, 2), (4, 1, 2), (4, 1, 3), (4, 2, 1), (4, 2, 3), (4, 3, 1), (4, 3, 2)]

import itertools

# 从列表中选取3个元素进行组合
items = [1, 2, 3, 4]
combinations = list(itertools.combinations(items, 3))
print(combinations)

[(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]