为什么我在这段代码中收到“UnboundLocalError”？

Question

I am getting UnboundLocalError: local variable 'prev' referenced before assignment on the given code below

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

class MyList:
    def __init__(self,a):
        self.head=None
        tail=None
        for i in a:
            n=Node(i,None)
            if self.head is None:
                self.head=n
                tail=n
            else:
                tail.next=n
                tail=n
    
    def showList(self):
        n=self.head
        if n==None:
            print("Empty list")
        while n!=None:
            print(n.value)
            n=n.next

    def isEmpty(self):
        n=self.head
        if n is None:
            print("True")
        else:
            print("False")

    def clear(self):
        while(self.head!=None):
            temp=self.head
            self.head=self.head.next
            temp=None
    
    
    
    def insert(self, newElement):
        node = self.head
        while node:
            if node.value == newElement:
                print("Value already exists")
                return False  
            prev = node  
            node = node.next
    
        if prev:
            prev.next = Node(newElement)
        else:  
            self.head = Node(newElement)
        return True



list1 = [1, 2, 3, 4, 5]
linklist = MyList(list1)
linklist.showList()
linklist.isEmpty()
linklist.clear()
linklist.showList()
linklist.insert(6)  
linklist.showList()

If I remove the clear(self) function the code is working perfectly but when it is not removed an error is showing in the insert(self,newElement) function. It is showing

Traceback (most recent call last):
  File "c:\Users\acer\Desktop\Untitled-1.py", line 65, in <module>
    linklist.insert(6)
  File "c:\Users\acer\Desktop\Untitled-1.py", line 51, in insert
    if prev:
UnboundLocalError: local variable 'prev' referenced before assignment
PS C:\Users\acer>

Answer 1

In your insert method, you need to initialize prev to None before entering the loop. Otherwise, if the loop exits before the first iteration (ie, if self.head is None ), then prev will be undefined, which is what Python is telling you. Just add the initialization like so:

    def insert(self, newElement):]
        prev = None
        node = self.head
        while node:
            ...

Answer 2

In your insert() method if you do not enter the while loop then the prev variable will not be initialised.

What you need to do is initialise it to something at the start of your insert() method or ensure that you are always entering the while loop.