理解 Python 交换:为什么 a, b = b, a 并不总是等价于 b, a = a, b? [英] Understand Python swapping: why is a, b = b, a not always equivalent to b, a = a, b?

问题描述

众所周知，交换a和b两项值的pythonic方式是

As we all know, the pythonic way to swap the values of two items a and b is

a, b = b, a

它应该等价于

b, a = a, b

然而，今天我在做一些代码的时候，无意中发现以下两个交换给出了不同的结果:

However, today when I was working on some code, I accidentally found that the following two swaps give different results:

nums = [1, 2, 4, 3]
i = 2
nums[i], nums[nums[i]-1] = nums[nums[i]-1], nums[i]
print(nums)
# [1, 2, 4, 3]

nums = [1, 2, 4, 3]
i = 2
nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]
print(nums)
# [1, 2, 3, 4]

这对我来说令人难以置信.有人可以向我解释这里发生了什么吗?我认为在 Python 交换中，两个分配同时且独立地发生.

This is mind-boggling to me. Can someone explain to me what happened here? I thought in a Python swap the two assignments happen simultaneously and independently.

推荐答案

来自 python.org

将对象分配给目标列表(可选地括在圆括号或方括号中)的递归定义如下.

Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.

...

• Else:对象必须是与目标列表中的目标数量相同的可迭代对象，并且项目从左到右分配给相应的目标.

所以我认为这意味着你的任务

So I interpret that to mean that your assignment

nums[i], nums[nums[i]-1] = nums[nums[i]-1], nums[i]

大致相当于

tmp = nums[nums[i]-1], nums[i]
nums[i] = tmp[0]
nums[nums[i] - 1] = tmp[1]

(当然还有更好的错误检查)

(with better error-checking, of course)

而另一个

nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]

就像

tmp = nums[i], nums[nums[i]-1]
nums[nums[i] - 1] = tmp[0]
nums[i] = tmp[1]

所以在两种情况下都会首先评估右侧.但是然后左边的两块按顺序求值，求值后立即赋值.至关重要的是，这意味着左侧的第二项仅在第一项分配已经完成后进行评估.因此，如果您先更新 nums[i]，那么 nums[nums[i] - 1] 引用的索引不同于更新 nums[i]] 秒.

So the right-hand side is evaluated first in both cases. But then the two pieces of the left-hand side are evaluated in order, and the assignments are done immediately after evaluation. Crucially, this means that the second term on the left-hand side is only evaluated after the first assignment is already done. So if you update nums[i] first, then the nums[nums[i] - 1] refers to a different index than if you update nums[i] second.

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