python 小技巧 列表 for i in list: 和 for i in list[ : ]的区别

代码一：

def test(l):
    s = [1,2,3]
    for i in range(len(s)):
        for st in l:
            st = st + [s[i]]
            l.append(st)
    return l

if __name__ == "__main__":
    nums = [[],]
    rres = test(nums)
    print(rres)

输出：报错 MemoryError

 

代码二：

def test(l):
    s = [1,2,3]
    for i in range(len(s)):
        for st in l[:]:
            st = st + [s[i]]
            l.append(st)
    return l

if __name__ == "__main__":
    nums = [[],]
    rres = test(nums)
    print(rres)

输出：[[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]

 

表明：第一种相当于会在列表继续追加，导致for循环爆栈，第二种首先会提出所有元素。