Разберитесь с свопингом в Python: почему a, b = b, a не всегда эквивалентно b, a = a, b? ⇐ Python

[Гость]

Как мы все знаем, питонический способ поменять местами значения двух элементов

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a

and

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b

is

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a, b = b, a

and it should be equivalent to

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b, a = a, b

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

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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]

What is happening here? I thought in a Python swap the two assignments happen simultaneously and independently.

See also Multiple assignment and evaluation order in Python regarding the basic semantics of this kind of assignment.
See also Multiple assignment semantics regarding the effect and purpose of parentheses on the left-hand side of a multiple assignment.


Источник: https://stackoverflow.com/questions/681 ... t-to-b-a-a