CODEWITHSUNDEEP
pythonlinked-list
EvelynZ
EvelynZ

Reputation: 73

Linked List Assignment Mutation

I have a question about how mutation works in linked lists with binding assignment.

Suppose we have the following linked list data type in python:

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

Suppose we have an object a and bind curr to a:

a = ListNode(1, ListNode(2))
curr = a

Both are 1 -> 2 ->. Now if I try to mutate curr:

curr.next = ListNode(3)
curr = curr.next

Now a is 1 -> 3 -> while curr is 3 ->.

Here comes the part where I get confused. If I keep updating curr by:

curr.next = ListNode(4)
curr = curr.next

Now a is 1 -> 3 -> 4 -> while curr is 4 ->.

I'm wondering why would a be updated in this way?

Upvotes: 1

Views: 88

Answers (1)

jas
jas

Reputation: 10865

Because after

curr = a, curr and a are the same object.

And then, after

curr = curr.next, curr and a.next are the same object, which you can see by looking at the IDs (or even better, by going retro and drawing boxes and arrows with pen and paper :)

In [8]: id(curr)
Out[8]: 4509135032

In [9]: id(a.next)
Out[9]: 4509135032

Hence when you then modify curr.next, you are also modifying a.next.next.

Upvotes: 2

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