# Learn Classes and Objects by Building a Sudoku Solver - Step 75

### Tell us what’s happening:

This step is requiring me to join the items in row_str with spaces, then append row_str to board_str. I have submitted this code but it still says that it is incorrect. I am confused as I have tested the output of board_str and it looks fine.

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

# User Editable Region

def __str__(self):
board_str = ''
for row in self.board:
row_str = [str(i) if i else '*' for i in row]
row_str = ' '.join(row_str)
board_str += row_str

# User Editable Region

def find_empty_cell(self):
for row, contents in enumerate(self.board):
try:
col = contents.index(0)
return row, col
except ValueError:
pass
return None

def valid_in_row(self, row, num):
return num not in self.board[row]

def valid_in_col(self, col, num):
return all(self.board[row][col] != num for row in range(9))

def valid_in_square(self, row, col, num):
row_start = (row // 3) * 3
col_start = (col // 3) * 3
for row_no in range(row_start, row_start + 3):
for col_no in range(col_start, col_start + 3):
if self.board[row_no][col_no] == num:
return False
return True

def is_valid(self, empty, num):
row, col = empty
valid_in_row = self.valid_in_row(row, num)
valid_in_col = self.valid_in_col(col, num)
valid_in_square = self.valid_in_square(row, col, num)
return all([valid_in_row, valid_in_col, valid_in_square])

def solver(self):
if (next_empty := self.find_empty_cell()) is None:
return True
for guess in range(1, 10):
if self.is_valid(next_empty, guess):
row, col = next_empty
self.board[row][col] = guess
if self.solver():
return True
self.board[row][col] = 0
return False

def solve_sudoku(board):
gameboard = Board(board)
print(f'Puzzle to solve:\n{gameboard}')
if gameboard.solver():
print(f'Solved puzzle:\n{gameboard}')
else:
print('The provided puzzle is unsolvable.')
return gameboard

puzzle = [
[0, 0, 2, 0, 0, 8, 0, 0, 0],
[0, 0, 0, 0, 0, 3, 7, 6, 2],
[4, 3, 0, 0, 0, 0, 8, 0, 0],
[0, 5, 0, 0, 3, 0, 0, 9, 0],
[0, 4, 0, 0, 0, 0, 0, 2, 6],
[0, 0, 0, 4, 6, 7, 0, 0, 0],
[0, 8, 6, 7, 0, 4, 0, 0, 0],
[0, 0, 0, 5, 1, 9, 0, 0, 8],
[1, 7, 0, 0, 0, 6, 0, 0, 5]
]
``````

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