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

But the results are different.
Why are the results different? (why is the second one that result?)

Answer 1

Prerequisites - 2 important Points

• Lists are mutable

  The main part in lists is that lists are mutable. It means that the values of lists can be changed. This is one of the reason why you are facing the trouble. Refer the docs for more info

• Order of Evaluation

  The other part is that while unpacking a tuple, the evaluation starts from left to right. Refer the docs for more info

---

Introduction

when you do a,b = c,d the values of c and d are first stored. Then starting from the left hand side, the value of a is first changed to c and then the value of b is changed to d.

The catch here is that if there are any side effects to the location of b while changing the value of a, then d is assigned to the later b, which is the b affected by the side effect of a.

---

Use Case

Now coming to your problem

In the first case,

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

nums[0] is initially 1 and nums[nums[0]] is 2 because it evaluates to nums[1]. Hence 1,2 is now stored into memory.

Now tuple unpacking happens from left hand side, so

nums[nums[0]] = nums[1] = 1   # NO side Effect. 
nums[0] = 2

hence print nums will print [2, 1, 0]

However in this case

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

nums[nums[0]], nums[0] puts 2,1 on the stack just like the first case.

However on the left hand side, that is nums[0], nums[nums[0]], the changing of nums[0] has a side effect as it is used as the index in nums[nums[0]]. Thus

nums[0] = 2
nums[nums[0]] = nums[2] = 1  # NOTE THAT nums[0] HAS CHANGED

nums[1] remains unchanged at value 2. hence print nums will print [2, 2, 1]