Python: local variable not recognized

Question

I'm new to python and I'm trying to implement a function that reverses a link list.

I'm getting the error msg: local variable "new_head" referenced before assignment

def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if head == None or head.next == None:
            return head
        
        new_head = None
        
        def recur(curr_head):
            if curr_head == None:
                return
            
            next_node = curr_head.next
            curr_head.next = new_head
            new_head = curr_head
            recur(next_node)
            
        recur(head)
        return new_head

The following implementation works but is very ugly, is there an alternative to this?

def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if head == None or head.next == None:
            return head
        
        
        new_head = None
        arr = [new_head]
        
        def recur(curr_head):
            if curr_head == None:
                return
            
            temp = curr_head.next
            curr_head.next = arr[0]
            arr[0] = curr_head
            recur(temp)
            
        recur(head)
        return arr[0]

Answer 1

At the beginning of the function recur , you can put:

nonlocal new_head

This allows the inner function recur to modify a variable declared in the outer function's scope.

(It's like the way you need to use global to allow a function to modify a module-scope variable.)

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