问题描述:

My intention is to swap elements in the list, and it works:

arr = [4,3,2,1,0]
idx = arr[0]
arr[0], arr[idx] = arr[idx], arr[0]
print(arr)
>> [0, 3, 2, 1, 4]

However this does not:

arr = [4,3,2,1,0]
arr[0], arr[arr[0]] = arr[arr[0]], arr[0]
print(arr)
>> [4, 3, 2, 1, 0]

How python evaluates the latter example?

解决方案 1:^[1]

It’s easier to see what’s happening if you split the assignment into multiple statements.

arr[0], arr[arr[0]] = arr[arr[0]], arr[0]

is equivalent to:

temp = arr[arr[0]], arr[0] # temp = (0, 4)
arr[0] = temp[0]           # arr[0] = 0
arr[arr[0]] = temp[1]      # arr[0] = 4

Notice that arr[0] is being updated before we assign to arr[arr[0]]. That updated index is being used in the last assignment.

None of this ever reassigns arr[4], so it doesn’t swap the two elements.

Let’s use the walrus operator (:=) to gain some insight into what’s happening.

>>> arr = [4, 3, 2, 1, 0]
>>> arr[0], arr[a := arr[0]] = arr[b := arr[0]], arr[0]
>>> arr
[4, 3, 2, 1, 0]
>>> a
0
>>> b
4

Now if we rework this slightly:

>>> arr = [4,3,2,1,0]
>>> arr[0], arr[b] = arr[b := arr[0]], arr[0]
>>> print(arr)
[0, 3, 2, 1, 4]

What’s happening is similar to the unexpected behavior programmers see when modifying a collection while iterating over it.

Article Source: StackOverflow