The __str__ method fails to execute and an attribute error occurs.

# -*- coding: utf-8 -*-
"""
Created on Tue Sep 28 20:54:55 2021

@author: Salieli
"""

class State:
    def __init__(self, board, goal, moves=0):
        self.board = board
        self.moves = moves
        self.goal = goal

        # Exchange i1 and i2 to return a new state
        def get_new_board(self, i1, i2, moves):
            new_board = self.board[:]
            new_board[i1], new_board[i2] = new_board[i2], new_board[i1]

            return State(new_board, self.goal, moves)

        # To expand and save a child node to a list and return it
        def expand(self, moves):
            result = []
            i = self.board.index(0)
            if not i in [0,1,2]:# up
                    result.append(self.get_new_board(i,i-3,moves))          
                    if not i in [0,3,6]:# LEFT
                       result.append(self.get_new_board(i,i-1,moves))
                    if not i in [2,5,8]:# RIGHT 
                       result.append(self.get_new_board(i,i+1,moves))
                    if not i in [6,7,8]:# DOWN
                       result.append(self.get_new_board(i,i+3,moves))
            return result


        def __str__(self):
            return str(self.board[:3])+"\n"+\
                str(self.board[3:6])+"\n"+\
                str(self.board[6:])+"\n"+\
                "-----------------"

        def __eq__(self,other):
               return self.board == other.board

# Initial state
puzzle = [2,8,3,
          1,0,4,
          7,6,5]

# Target Status
goal = [1,2,3,
        4,5,6,
        7,8,0]

# # open list
open_queue = [] 
open_queue.append(State(puzzle, goal)) 

# # closed list
closed_queue = []
moves = 0;

while len(open_queue) !=0:
    current = open_queue.pop(0)
    print(current)    
    if current.board == goal:
        print ("Explore Success")
        break    
    moves = current.moves+1             
    closed_queue.append(current)

    for state in current.expand(moves): 
        if (state in closed_queue) or (state in open_queue):
            continue
        else:
            open_queue.append(State)
            print ("Discovery Failed")