Is assigning a,b = c,a the same as b,a = a,c?

Question

Is assigning

a,b = c,a

the same as

b,a = a,c

It seems to be the same for the simple case where a , b , and c are numbers.

Is there a case where this fails?

Answer 1

No, there is no case where this fails. The two statements are exactly equivalent, because a and b are independent .

That's because the right-hand side values are put on the stack first before assignment takes place (from left to right). Assigning to a before b makes no difference, because a and b are entirely different names, and assignment to either one first won't affect the other.

So the first statement:

a,b = c,a

is executed as (psuedo-bytecode ^* )

PUSH C ON STACK
PUSH A ON STACK
SWAP TOP TWO VALUES OF STACK
POP TOP OF STACK, STORE IN A
POP TOP OF STACK, STORE IN B

while the second

b,a = a,c

is executed as

PUSH A ON STACK
PUSH C ON STACK
SWAP TOP TWO VALUES OF STACK
POP TOP OF STACK, STORE IN B
POP TOP OF STACK, STORE IN A

It doesn't matter what type the values referenced by a , b or c are.

Things can get more confusing if you were to chain assignments and / or mix in subscriptions . In all those cases the names being assigned to are not independent, and people usually don't realise that all assignment takes place after the right-hand side expression has been executed and that assignment takes place from left to right after that.

---

^* See How does swapping of members in the python tuples (a,b)=(b,a) work internally? for actual bytecode examples.

Answer 2

It's the same in all cases, no matter what you're working with. So long as statements are equivalent (like yours are), the result will be the same.

Answer 3

简短答案-----“是” .. a，b = c，a表示a = c和b = a ... b，a = a，c表示b = a和a = c