Python對list去重的各類方法

參考原文：https://www.the5fire.com/python-remove-duplicates-in-list.html

需求：去list進行去重，去重後保證順序不變

方法1：for循環

ids = [1, 2, 3, 3, 4, 2, 3, 4, 5, 6, 1]
new_ids = []

for id in ids:
    if id not in new_ids:
        new_ids.append(id)

print("new_ids==>", new_ids)

方法2：set

ids = [1,4,3,3,4,2,3,4,5,6,1]
new_ids = list(set(ids))

print(new_ids)

測試發現去重後不能保證原來的順序

方法3：按照索引再次排序

ids = [1, 4, 3, 3, 4, 2, 3, 4, 5, 6, 1]
new_ids = list(set(ids))
new_ids.sort(key=ids.index)

print(new_ids)

方法4：用reduce

ids = [1,4,3,3,4,2,3,4,5,6,1]
func = lambda x,y:x if y in x else x + [y]
reduce(func, [[], ] + ids)
[1, 4, 3, 2, 5, 6]

其中的 lambda x,y:x if y in x else x + [y] 等價於 lambda x,y: y in x and x or x+[y] 。
思路其實就是先把ids變爲[[], 1,4,3,......] ,而後在利用reduce的特性

reduce()函數介紹