solver.py

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

# function to format board printing to the console
def print_board(board):
    for i in range(len(board)):
        if i % 3 == 0 and i != 0:
            print("- - - - - - - - - - - ")

        for j in range(len(board[0])):
            if j % 3 == 0 and j != 0:
                print(" | ", end = "")

            if j == 8:
                print(board[i][j])
            else:
                print(str(board[i][j]) + " ", end = "")

# function to pick empty squares
def find_empty(board):
    for i in range(len(board)):
        for j in range(len(board[0])):
            if board[i][j] == 0:
                return (i, j) # returns row and column respectively
    return None

# function to try all numbers and find a number that works in a particular position
def is_valid(board, number, position):
    # check for row
    for i in range(len(board[0])):
        if board[position[0]][i] == number and position[1] != i:
            return False

    # check for column
    for i in range(len(board)):
        if board[i][position[1]] == number and position[0] != i:
            return False

    # check for the 3 X 3 cubes
    box_x = position[1] // 3
    box_y = position[0] // 3

    for i in range(box_y * 3, box_y * 3 + 3):
        for j in range(box_x * 3, box_x * 3 + 3):
            if board[i][j] == number and (i,j) != position:
                return False

    return True

# function to perform the backtracking
def solve(board):
    # Recursion base case
    find = find_empty(board)
    if not find:
        return True
    else:
        row, column = find

    for i in range(1, 10):
        if is_valid(board, i, (row, column)):
            board[row][column] = i

            if solve(board):
                return True

            board[row][column] = 0

    return False


print_board(starting_board)
solve(starting_board)
print("________________________ \n")
print_board(starting_board)