使用 Pythonic 变量交换迭代地反转链表

Question

I was just practicing some Linked List questions when I came across something I couldn't explain. So I was wondering if anyone could help me out.

If I have a Linked List with nodes implemented as follows

class Node:
    def __init__(self, val, next):
        self.val = val
        self.next = next

I tried reversing the Linked List initially like so and it came up with an AttributeError: 'NoneType' object has no attribute 'next'

prev = None
curr = head
while curr:
    prev, curr, curr.next = curr, curr.next, prev

And then I modified it to and it worked but I can't seem to wrap my head around it.

prev = None
curr = head
while curr:
    curr.next, prev, curr = prev, curr, curr.next

I'm aware that Python evaluates the RHS as a tuple and then assigns the values on the LHS from left to right but I can't seem to understand why both code snippets wouldn't be the same if this is the case.

Answer 1

You might want to visualize things the following way:

Your first code is equivalent to:

prev = None
curr = head
while curr:
    tmp = curr, curr.next, prev
    prev = tmp[0]
    curr = tmp[1]
    curr.next = tmp[2]

whereas your second code is equivalent to:

prev = None
curr = head
while curr:
    tmp = prev, curr, curr.next
    curr.next = tmp[0]
    prev = tmp[1]
    curr = tmp[2]

you see the difference is, that in one case you first change curr and then you assign to curr.next , whereas in the other case you first change curr.next and then reassign curr .

python executes nothing in parallel.

It has to chose an order which is left to right for the variables to be assigned to.