Solves sudoku puzzles

sudoku_solver.py

import itertools

ROWS = 9
COLS = 9
CELLS = ROWS * COLS


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


def solve_board(ROWS, COLS, board):
    # Print the board
    print("Initial board:")
    print("-" * 25)
    for i in range(ROWS):
        row = "| "
        for j in range(COLS):
            row += str(board[i][j]) + " "
            if (j + 1) % 3 == 0:
                row += "| "
        print(row)
        if (i + 1) % 3 == 0:
            print("-" * 25)
    print()

    # Solve the board
    solved = False
    while not solved:
        solved = True
        for i, j in itertools.product(range(ROWS), range(COLS)):
            if board[i][j] == 0:
                solved = False
                possible = set(range(1, 10))
                for k in range(ROWS):
                    possible.discard(board[i][k])
                    possible.discard(board[k][j])
                for k, l in itertools.product(range(3), range(3)):
                    possible.discard(board[3 * (i // 3) + k][3 * (j // 3) + l])
                if len(possible) == 1:
                    board[i][j] = possible.pop()
    print("Solved board:")
    print("-" * 25)
    for i in range(ROWS):
        row = "| "
        for j in range(COLS):
            row += str(board[i][j]) + " "
            if (j + 1) % 3 == 0:
                row += "| "
        print(row)
        if (i + 1) % 3 == 0:
            print("-" * 25)
    print()


solve_board(ROWS, COLS, board)