为什么在 Python 中调用函数和编写语句的工作方式不同？

Question

I need an insert to head operation for a linked list that I implemented. However, doing this operation by function call (like insertToHead) and writing the statements explicitly where I need them produces different results. I wonder which property of Python leads to that difference but I couldn't figure it out.

To be more specific, let's say that I have the following class for the linked list:

class Node:
    value = None
    nextNode = None

    def __init__(self, value):
        self.value = value

    def insertToHead(self, value):
        newHead = Node(value)
        newHead.nextNode = self
        return newHead

For a linked list with a single element (say, 2) I want to insert a node (say, 0) to the head to make linked list 0 -> 2.

I created the linked list the following way

head = Node(2)

Then I tried to insert 0 to head two ways:

1. Writing the statements explicitly where I need them

newHead = Node(0)
newHead.next = head
head = newHead

Now head is 0, not 0 -> 2.

2. Calling insertToHead

head = head.insertToHead(0)

head is 0 -> 2 after this statement.

Does anyone know why these two approaches result in differently?

Answer 1

You have a typo. newHead.next should be newHead.nextNode .

Answer 2

A simple implementation of Singly Linked Lists:

class Node:
    def __init__(self, value = None, nextNode = None):
        self.value = value
        self.nextNode = nextNode

class LinkedList:
    def __init__(self):
        self.head = None  # will point to the head of the list
        self.tail = None  # will point to the tail of the list
        self.size = 0     # size of the linked list

    def insert_to_head(self, data):
        # when push front, the head of the linked list will be Node()
        self.head = Node(data, self.head)
        if self.tail == None:  # if tail is None, means it is a single element
            self.tail = self.head
        self.size += 1  # increase size by one


    def __str__(self):
        ret_str = ""
        node = self.head
        while node != None:
            ret_str += str(node.value) + " -> "
            node = node.nextNode
        return ret_str




myLinkedList = LinkedList()
myLinkedList.insert_to_head(3)
myLinkedList.insert_to_head(2)

print(myLinkedList)