_9_sudoku_solver_simpler.py

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

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

    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
        else:
            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'\nPuzzle to solve:\n{gameboard}')
    if gameboard.solver():
        print('\nSolved puzzle:')
        print(gameboard)
    else:
        print('\nThe provided puzzle is unsolvable.')
    return gameboard


if __name__ == '__main__':
    puzzle = [
        [1, 0, 0, 2, 0, 0, 3, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
        [0, 0, 0, 0, 0, 0, 0, 0, 0],
    ]
    solve_sudoku(puzzle)