在 python3 中交换列表中的两个元素时出现非常奇怪的行为
very strange behavior when swap two elements in the list in python3
我有一个列表
a = [-1,4,3,1]
我们有
a[1] = 4
a[3] = 1
现在我想交换 a[1] 和 a[3]
以下代码运行良好：
a[1], a[3] = a[3], a[1]
但是如果我使用：
a[1], a[a[1]-1] = a[a[1]-1], a[1]
它给了我：
a = [4,1,3,1]
答案不正确。自从我们知道 a[1] - 1 = 3 以来，这里发生了什么。任何想法，谢谢。
这与 Multiple assignment and evaluation order in Python 基本上是同一个问题，但由于复杂的索引而出现了表面皱纹。
注意事项：
右侧在左侧完全被评估之前被完全评估
元组赋值完全从左到右求值
文档在这里（尽管很少）说明了这一点：https://docs.python.org/3/reference/expressions.html#evaluation-order
Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.
在这里，我将根据您的情况说明这些规则：
State of a Evaluation of expression Comment
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[a[1]-1], a[1] <start>
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[4-1], a[1] a[1] resolves to 4
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[3], a[1] 4-1 is 3
[-1, 4, 3, 1] a[1], a[a[1]-1] = 1, a[1] a[3] resolves to 1
[-1, 4, 3, 1] a[1], a[a[1]-1] = 1, 4 a[1] resolves to 4
[-1, 1, 3, 1] a[a[1]-1] = 4 a[1] = 1 assigned
[-1, 1, 3, 1] a[1-1] = 4 a[1] resolves to 1
[-1, 1, 3, 1] a[0] = 4 1-1 is 0
[4, 1, 3, 1] a[0] = 4 assigned
我有一个列表
a = [-1,4,3,1]
我们有
a[1] = 4
a[3] = 1
现在我想交换 a[1] 和 a[3]
以下代码运行良好：
a[1], a[3] = a[3], a[1]
但是如果我使用：
a[1], a[a[1]-1] = a[a[1]-1], a[1]
它给了我：
a = [4,1,3,1]
答案不正确。自从我们知道 a[1] - 1 = 3 以来，这里发生了什么。任何想法，谢谢。
这与 Multiple assignment and evaluation order in Python 基本上是同一个问题，但由于复杂的索引而出现了表面皱纹。
注意事项：
右侧在左侧完全被评估之前被完全评估
元组赋值完全从左到右求值
文档在这里（尽管很少）说明了这一点：https://docs.python.org/3/reference/expressions.html#evaluation-order
Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.
在这里，我将根据您的情况说明这些规则：
State of a Evaluation of expression Comment
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[a[1]-1], a[1] <start>
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[4-1], a[1] a[1] resolves to 4
[-1, 4, 3, 1] a[1], a[a[1]-1] = a[3], a[1] 4-1 is 3
[-1, 4, 3, 1] a[1], a[a[1]-1] = 1, a[1] a[3] resolves to 1
[-1, 4, 3, 1] a[1], a[a[1]-1] = 1, 4 a[1] resolves to 4
[-1, 1, 3, 1] a[a[1]-1] = 4 a[1] = 1 assigned
[-1, 1, 3, 1] a[1-1] = 4 a[1] resolves to 1
[-1, 1, 3, 1] a[0] = 4 1-1 is 0
[4, 1, 3, 1] a[0] = 4 assigned