Python链表delete函数调试及insert方法修复方案

我正在学习一门包含Python链表操作的在线课程，课程要求为LinkedList类实现get_position、insert和delete方法，并运行脚本底部的测试用例得到预期结果。我卡在delete函数上数日，未能找到问题所在。

"""The LinkedList code from before is provided below.
Add three functions to the LinkedList.
"get_position" returns the element at a certain position.
The "insert" function will add an element to a particular
spot in the list.
"delete" will delete the first element with that
particular value.
Then, use "Test Run" and "Submit" to run the test cases
at the bottom."""

class Element(object):
    def __init__(self, value):
        self.value = value
        self.next = None
        
class LinkedList(object):
    def __init__(self, head=None):
        self.head = head
        
    def append(self, new_element):
        current = self.head
        if self.head:
            while current.next:
                current = current.next
            current.next = new_element
        else:
            self.head = new_element
            
    def get_position(self, position):
        """Get an element from a particular position.
        Assume the first position is "1".
        Return "None" if position is not in the list."""
        current = self.head
        i = 1
        if self.head:
            while i < position:
                current = current.next
                i = i + 1
            return current
        else:
            return None
    
    def insert(self, new_element, position):
        """Insert a new node at the given position.
        Assume the first position is "1".
        Inserting at position 3 means between
        the 2nd and 3rd elements."""
        current = self.head
        i = 1
        if self.head:
            while i < position - 1:
                current = current.next
                i = i + 1
            current.next = new_element
            while current.next:
                current = current.next
        else:
            self.head = new_element
        pass
    
    
    def delete(self, value):
        """Delete the first node with a given value."""
        if self.head is None:
            raise Exception("List is empty")
        
        if self.head.value == value:
            self.head = self.head.next
            current = self.head
            i = 1
            while current.next:
                current = current.next
            return
        
        previous = self.head
        for element in self:
            if element.value == value:
                previous.next = element.next
                return
            previous = element

# Test cases
# Set up some Elements
e1 = Element(1)
e2 = Element(2)
e3 = Element(3)
e4 = Element(4)

# Start setting up a LinkedList
ll = LinkedList(e1)
ll.append(e2)
ll.append(e3)

# Test get_position
# Should print 3
print ll.head.next.next.value
# Should also print 3
print ll.get_position(3).value

# Test insert
ll.insert(e4,3)
# Should print 4 now
print ll.get_position(3).value

# Test delete
ll.delete(1)
# Should print 2 now
print ll.get_position(1).value
# Should print 4 now
print ll.get_position(2).value
# Should print 3 now
print ll.get_position(3).value

Traceback (most recent call last): 
  File "vm_main.py", line 31, in <module> 
    import main 
  File "/tmp/workspaces/04497b9c-ee1c-47bc-9e15-b58ef0e5580c/main.py", line 2, in <module> 
    import studentMain 
  File "/tmp/workspaces/04497b9c-ee1c-47bc-9e15-b58ef0e5580c/studentMain.py", line 2, in <module> 
    import algorithmsP2 
  File "/tmp/workspaces/04497b9c-ee1c-47bc-9e15-b58ef0e5580c/algorithmsP2.py", line 112, in <module> 
    print ll.get_position(3).value 
AttributeError: 'NoneType' object has no attribute 'value'

问题实际出在insert方法，而非最初以为的delete方法。原insert方法直接将current.next设为新元素，丢失了原本指向后续节点的引用，导致链表断裂。修改后的insert方法先保存原本的后续节点，再将新元素的next指向该节点，最后更新current.next为新元素，保证链表结构完整。

修改后的insert方法代码：

def insert(self, new_element, position):
    """Insert a new node at the given position.
    Assume the first position is "1".
    Inserting at position 3 means between
    the 2nd and 3rd elements."""
    current = self.head
    i = 1
    if self.head:
        while i < position - 1:
            current = current.next
            i = i + 1
        that = current.next
        new_element.next = that
        current.next = new_element
        while current.next:
            current = current.next
    else:
        self.head = new_element

内容的提问来源于stack exchange，提问作者Adxs