python - Python 多重赋值会引发错误,但单独赋值不会
问题描述
我正在反转一个链表,但是多个分配会破坏这个功能,而单独的分配不会。有人可以解释这两个代码部分之间的执行差异吗?
我知道表达式的右侧是在赋值之前评估的,但是据我所知,如果是这种情况,我将无处访问 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
相当于
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
有人可以解释这两个代码部分之间的执行差异吗?
解决方案
我还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示例:
class A:
def __init__(self, a):
self.a = a
a = A(1)
a, a.a = a.a, None
这些问题是,虽然右侧是一次性评估的,但左侧仍然是从左到右评估的。这意味着a
将a.a
完美地重新分配,但a.a
在左侧被评估时,a
将不再具有该.a
属性,因为它已被重新分配给一个 int。
用以下内容替换最后一行具有相同的结果,并且可以更清楚地说明问题:
t = a.a, None
a = t[0]
a.a = t[1]
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