Learn Classes and Objects by Building a Sudoku Solver - Step 14

I think I’m doing the right thing , but my code won’t pass. Can someone please help correct the error?

class Board:
    def __init__(self, board):
        self.board = board

    def __str__(self):
        upper_lines = f'\n╔═══{"╤═══"*2}{"╦═══"}{"╤═══"*2}{"╦═══"}{"╤═══"*2}╗\n'
        middle_lines = f'╟───{"┼───"*2}{"╫───"}{"┼───"*2}{"╫───"}{"┼───"*2}╢\n'
        lower_lines = f'╚═══{"╧═══"*2}{"╩═══"}{"╧═══"*2}{"╩═══"}{"╧═══"*2}╝\n'
        board_string = upper_lines

        for index, line in enumerate(self.board):
            row_list = []

# User Editable Region

            for square_no, part in enumerate([line[:3], line[3:6], line[6:]], start=1):
                for item in part():
                    item.join("|")
                    str(item)

                

# User Editable Region

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Challenge Information:

Learn Classes and Objects by Building a Sudoku Solver - Step 14

You are getting this hint:

You should use the join() method on the | character to join the elements of the segment (part). add test for “” as well

I think it’s telling you to call the join method on the "|" (not item)