Tuple unpacking order changes values assigned

Question

I think the two are identical.

nums = [1, 2, 0]    
nums[nums[0]], nums[0] = nums[0], nums[nums[0]]    
print nums  # [2, 1, 0]

nums = [1, 2, 0]    
nums[0]         


        

Answer 1

It's because of that python assignment priority is left to right.So in following code:

 nums = [1, 2, 0]
 nums[nums[0]], nums[0] = nums[0], nums[nums[0]]

It first assigned the nums[0] to nums[nums[0]] means nums[1]==1 and then since lists are mutable objects the nums would be :

[1,1,0]

and then nums[nums[0]] will be assigned to nums[0] which means nums[0]==2 and :

nums = [2,1,0]

And like so for second part.

Note that the important point here is that list objects are mutable and when you change it in a segment of code it can be change in-place. thus it will affect of the rest of the code.

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.