英文:
Appending a value to a list inside a list
问题
以下是您要翻译的代码部分：
list = [1,2,3,4]
for x in [list] #Yes a list in a list:
x.append(2)
print(x)
print(list)
翻译结果：
列表 = [1,2,3,4]
for x in [列表] #是的，一个列表在另一个列表中：
x.append(2)
print(x)
print(列表)
英文:
A simple question regarding the appending rule of list.
So the code is
list = [1,2,3,4]
for x in [list] #Yes a list in a list:
x.append(2)
print(x)
print(list)
The result of the two print is both [1,2,3,4,2]. I can see why print(x) outputs 1,2,3,4,2 but I didnt update the list yet but somehow the list is also updated.
Is there a hidden rule behind this? or is it just a default setting I need to memorize.
Thanks
Two prints have the same output and I wanna know why
答案1
得分: 1
在Python中，列表是按引用传递的，而不是按值传递的。
[list] 中的内部列表引用的是外部 list 中的相同列表。
因此，您进行的附加操作在两个引用中都可见（它们引用的是同一个列表）。
我制作了一个更简单的示例来说明同一点：
a = [1, 2, 3]
b = [a]
b[0].append(4)
print(a)
# 输出: [1, 2, 3, 4]
英文:
Lists in python are by reference, not by value.
inner list in [list] is referring to the same list as outside list
Therefore, the append you do is visible in both references (they are the same list).
I've made a simpler example that illustrates the same point:
a = [1, 2, 3]
b = [a]
b[0].append(4)
print(a)
>>> [1, 2, 3, 4]
答案2
得分: 0
这是类似于你的代码，我将其拆分以更好地解释：
sublist = [1,2,3,4]
sublist 是一个简单的一维数组。
li = [sublist]
li 是一个包含 sublist 的列表，实际上是一个二维数组。
for x in li:
如果你迭代 li，x 是li的每个元素，所以在最终迭代中，x 将是列表 [1,2,3,4]。
x.append(2)
然后，你将2附加到 x，实际上是将其附加到 sublist。
完整的代码如下：
sublist = [1,2,3,4]
li = [sublist]
print(li) # => [[1, 2, 3, 4]]
for x in li:
print(x) # => [1, 2, 3, 4]
x.append(2) # 将2附加到x（即 [1, 2, 3, 4] <= 2），这包含在li中
print(x) # => [1, 2, 3, 4, 2]
print(li) # => [[1, 2, 3, 4, 2]]
英文:
Here is a similar code to yours, I broken down to explain it better:
sublist = [1,2,3,4]
sublist is a simple 1d array.
li = [sublist]
li is a list containing sublist, effectivly a 2d array.
for x in li:
If you iterate over li, x is each element of li, so first a final iteration, x will be the list [1,2,3,4].
x.append(2)
Then you append 2 to x so you are effectivly appending to sublist.
The complete code looks like this:
sublist = [1,2,3,4]
li = [sublist]
print(li) # => [[1, 2, 3, 4]]
for x in li:
print(x) # => [1, 2, 3, 4]
x.append(2) # appending 2 to x (i.e [1, 2, 3, 4] <= 2) that is contained in li
print(x) # => [1, 2, 3, 4, 2]
print(li) # => [[1, 2, 3, 4, 2]]
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