how does python assignment(=) operator work

Question

I write a linked list，when I use multiple assignment ,the result is strange。What's the different between python and javascript?

The sample python code

class ListNode(object):
def __init__(self, x):
    self.val = x
    self.next = None

node=ListNode(0)
node=node.next=ListNode(10)
#expect node.next=ListNode(10) node=node.next
print(node ==node.next) # True why?

The same logic JavaScript code

function ListNode(val){
    this.val=val;
    this.next=null;
}
var node=new ListNode(0);
node=node.next=new ListNode(10)
console.log(node==node.next) //false

Thyrst' · Accepted Answer · 2017-04-09T13:49:25.197

1

In Python, evaluation is in reverse order. Therefore this:

node = node.next = ListNode(10)

is the same as this:

node = ListNode(10)
node.next = node

So, you have to reverse order of elements before last assignment:

node.next = node = ListNode(10)
# same as:
node.next = ListNode(10)
node = node.next

edited Apr 09 '17 at 13:49

answered Apr 08 '17 at 19:37

Thyrst'

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The assignment `node.next = node = ListNode(10)` is the equivalent of `tmp = ListNode(10); node.next = tmp; node = tmp`, with the tmp variable hidden. The answer as written suggests that `node` isn't changed by the assignment, which isn't correct. – Craig Apr 09 '17 at 13:37
@Craig Thanks. Fixed. :) – Thyrst' Apr 09 '17 at 13:50
@Craig where can I know the assignment detail,I can't find in the Python Tutorial – shenli Apr 10 '17 at 02:34
@shenli - It's in the [documentation](https://docs.python.org/3/reference/simple_stmts.html#assignment-statements) _An assignment statement evaluates the expression list ... and assigns the single resulting object to each of the target lists, **from left to right**_ (emphasis mine). I would avoid using this construction for anything other than simple initialization. You see how much confusion it can cause. H/T: https://stackoverflow.com/questions/14056146/status-of-chained-assignment-in-python – Craig Apr 10 '17 at 02:42

score 0 · Answer 2 · answered Apr 08 '17 at 18:29

0

Actually you're reassigning node to be equal to node.next, so node == node.next will always be True because those two variables are pointing at the same object.

answered Apr 08 '17 at 18:29

lch

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score 0 · Answer 3 · answered Apr 08 '17 at 19:46

I'm not sure what you are trying to accomplish with the line:

node = node.next = ListNode(10)

If you just do:

node.next = ListNode(10)

You will get the result you want

in [1]: node=ListNode(0)

In [2]: node.val
Out[2]: 0

In [3]: node.next = ListNode(10)

In [4]: node.val
Out[4]: 0

In [5]: node.next.val
Out[5]: 10

In [6]: node == node.next
Out[6]: False

score -1 · Answer 4 · answered Apr 08 '17 at 18:25

-1

The assignment operation is evaluated from right to left:

node.next = ListNode(10)
node = node.next

answered Apr 08 '17 at 18:25

Daniel

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This is no what's happening! – Thyrst' Apr 08 '17 at 19:24

how does python assignment(=) operator work

4 Answers4