I'm trying to understand some code that reverses a linked list.
Here's how I'm constructing a linked list:
class Node:
def __init__(self, value, next=None):
self.value = value
self.next = next
def initialize():
start = Node(0)
prev = start
for i in range(1, 6):
cur = Node(i)
prev.next = cur
prev = cur
return start
Here's the correct code in the answer:
def reverse_list(head):
new_head = None
while head:
head.next, head, new_head = new_head, head.next, head
return new_head
And here's what I have:
def reverse_list(head):
new_head = None
while head:
new_head, head, head.next = head, head.next, new_head
return new_head
However, my code throws an AttributeError when reversing a linked list:
AttributeError: 'NoneType' object has no attribute 'next'
I'm using the exactly same replacements as the correct answer, only in a different order. Why does my code throw an error? Does order matter when it comes to one line variable reassignments?
The problem is that head.next becomes None after the first iteration. Then on the second iteration head gets assigned to head.next i.e. is set to None before calling head.next (on the left side) hence the error. For instance let's have the following scenario:
class Spam:
def __init__(self):
self.foo = 1
self.bar = 2
spam = Spam()
spam.foo, spam.bar = spam.bar, spam.foo
print(spam.foo, spam.bar) # 2, 1 as expected
but when we do:
spam.foo, spam, spam.bar = spam.bar, None, spam.foo
results in:
Traceback (most recent call last): File "main.py", line 9, in spam.foo, spam, spam.bar = spam.bar, None, spam.foo AttributeError: 'NoneType' object has no attribute 'bar'
So to answer your question the order does matter. In fact changing the order of the toy example to:
spam.foo, spam.bar, spam = spam.bar, spam.foo, None
does not throw any exceptions.
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