Unable to comprehend this behaviour of swapping statement in Python

Question

I was solving a question on leetcode. "41. First Missing Positive"

For swapping, when I used

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

I got TLE (time limit exceeded).

But

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

is working perfectly.

This is not just the case with leetcode.

I tried to understand the difference, spent 1 hour on the problem. And then reached out to Google bard and Chatgpt. Both struggled.

After spending lots of time, I got an answer "Given that both codes share the same logic and conditional statements, it remains puzzling why Code 1 is exhibiting an issue, especially in the scenario you described with the infinite loop."

Here is my code:

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        i,n = 0,len(nums)

        while i<n:
            if nums[i]>0 and nums[i]<=n and nums[nums[i]-1]!=nums[i]:
                #nums[i],nums[nums[i]-1] = nums[nums[i]-1],nums[i]
                nums[nums[i]-1],nums[i] = nums[i],nums[nums[i]-1]

            else:
                i+=1
        for i in range(n):
            if nums[i]!=i+1:
                return i+1
        return n+1
        

The commented out statement is giving TLE , but second one is working fine. Can someone please explain, what is causing this? Is a,b=b,a and b,a=a,b different?

I also defined the function in Python IDLE, added a print statement below the swapping statement, and it got stuck in infinite loop again.

Defined function in Python IDLE

Output (infinite loop)

TLE with first statement

Accepted with second statement

Answer 1

Yes, the order in which the assignment happens changes.

In the first case,

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

First, nums[i] gets updated, then, nums[num[i] -1] gets updated, so that second assignment target is affected by the first change.

So it is equivalent to something like:

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

In the second case,

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

First nums[nums[i]-1] gets updated, then nums[i] gets updated. The nums[i] inside nums[nums[i]-1] will be different than the first case.

So, if you look at the documentation for assignment statements they give a simple example of what is going on:

Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example a, b = b, a swaps two variables), overlaps within the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints [0, 2]:

x = [0, 1]
i = 0
i, x[i] = 1, 2         # i is updated, then x[i] is updated
print(x)