python - Python 3 中列表内容发生意外更改

Question

我正在构建一个示例来创建一个列表的所有子集: 我的代码如下，我将结果值复制到另一个临时列表并在临时列表中进行操作，但不知道为什么在步骤 1 和步骤 3 中我打印的“结果”不同。我还没有做任何改变。

a=[1,2,3]

result=[]
temp=[]


def sub_sets(i,result):
    print("1start result:",result)
    temp=result[:]
    for j in range(len(temp)):
        temp[j].append(i)
    temp.append([i])
    print("2temp:",temp)
    print("3 middle result:",result)
    result.extend(temp)
    print("4end result:",result)


for i in range(len(a)):
    sub_sets(a[i],result)

Results:
1start result: []
2temp: [[1]]
3 middle result: []
4end result: [[1]]
1start result: [[1]]
2temp: [[1, 2], [2]]
3 middle result: [[1, 2]]
4end result: [[1, 2], [1, 2], [2]]
1start result: [[1, 2], [1, 2], [2]]
2temp: [[1, 2, 3, 3], [1, 2, 3, 3], [2, 3], [3]]
3 middle result: [[1, 2, 3, 3], [1, 2, 3, 3], [2, 3]]
4end result: [[1, 2, 3, 3], [1, 2, 3, 3], [2, 3], [1, 2, 3, 3], [1, 2, 3, 3],  [2, 3], [3]]         

Answer 1

Result 和 temp(函数的参数)是全局变量，因此需要特殊处理。下面的代码没有这个问题:

a=[1,2,3]
def sub_sets(i,result):
    print("1start result:",result)
    temp=result[:]
    for j in range(len(temp)):
        temp[j].append(i)
    temp.append([i])
    print("2temp:",temp)
    print("3 middle result:",result)
    result.extend(temp)
    print("4end result:",result)

for i in range(len(a)):
    result1=[]
    sub_sets(a[i],result1)

输出:

1start result: []
2temp: [[1]]
3 middle result: []
4end result: [[1]]
1start result: []
2temp: [[2]]
3 middle result: []
4end result: [[2]]
1start result: []
2temp: [[3]]
3 middle result: []
4end result: [[3]]