Python 多重赋值会引发错误，但单独赋值不会

Question

我正在反转一个链表，但是多个赋值会破坏这个功能，而单独的赋值不会。有人可以解释这两个代码段之间的执行差异吗？

我知道表达式的右侧是在赋值之前进行评估的，但是据我所知，如果是这种情况，我将无处可访问 None.next。

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

def clear():
    print("------------------------------------------")

def isPalindrome(A):
    if A is None:
        return True

    # get length
    cur, length = A, 1
    while cur.next is not None:
        cur = cur.next
        length += 1

    # go to second half
    cur, index = A, 0
    while index < (length + 1) / 2:
        cur = cur.next
        index += 1


    # start reversing
    prev = None
    while cur is not None:
        # this throws error? what is the difference?
        # cur, prev, cur.next = cur.next, cur, prev
        temp = cur.next
        cur.next = prev
        prev = cur
        cur = temp

    # now prev has reversed second half
    secondHalf = prev
    firstHalf = A

    # traverse both halves, comparing Ll[n] with Ll[length - 1 - n]
    while secondHalf is not None and firstHalf is not None:
        if firstHalf.val != secondHalf.val:
            return False

        firstHalf = firstHalf.next
        secondHalf = secondHalf.next

    return True

# even length simple case
def isPalindromeTest():
    A = Node(1, Node(1, Node(1, Node(1))))

    clear()
    print(isPalindrome(A))

isPalindromeTest()

我希望注释行

# cur, prev, cur.next = cur.next, cur, prev

等价于

temp = cur.next
cur.next = prev
prev = cur
cur = cur.next

如果我使用第一个代码部分，则会出现错误消息：

Traceback (most recent call last):
  File "linked_lists.py", line 1089, in <module>
    isPalindromeTest()
  File "linked_lists.py", line 1052, in isPalindromeTest
    print(isPalindrome(A))
  File "linked_lists.py", line 1027, in isPalindrome
    cur, prev, cur.next = cur.next, cur, prev
AttributeError: 'NoneType' object has no attribute 'next'

Implying that I am accessing None.next, which in this context it would have to be prev.next if anything?

第二段代码：

------------------------------------------
True

有人能解释一下这两个代码段的执行区别吗？

Answer 1

我也发现这种行为令人惊讶，但是一旦考虑到对象属性和执行顺序将如何在这样的实例中工作，它就很有意义。

为了自己演示，我做了一个MCVE 的例子：

class A:
    def __init__(self, a):
        self.a = a

a = A(1)
a, a.a = a.a, None

这些问题是虽然右侧是一次全部评估，但左侧仍然是从左到右评估。这意味着a 将完美地重新分配给a.a，但是在评估左侧的a.a 时，a 将不再具有.a 属性，因为它已被重新分配给一个int。

将最后一行替换为以下内容具有相同的结果，并且可以更清楚地说明问题：

t = a.a, None
a = t[0]
a.a = t[1]