main.py

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]
]

def solve(bo):  #backtracking algortithm
    find = find_empty(bo)
    if not find:
        return True
    else:
        row,col = find

    for i in range(1,10):
        if valid(bo,i,(row,col)):
            bo[row][col] = i
            if solve(bo):
                return True
            bo[row][col] = 0
    return False


def valid(bo, num, pos):  #checks if number is present in row/col already
    for i in range(len(bo[0])): #row checker
        if bo[pos[0]][i] == num and pos[1] != 1:
            return False

    for i in range(len(bo)): #column checker
        if bo[i][pos[1]] == num and pos[0] != i:
            return False

    box_x = pos[1] // 3
    box_y = pos[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 bo[i][j] == num and (i,j) != pos:
                return False
    return True

def print_board(bo):
    for i in range(len(bo)):
        if i % 3 == 0 and i != 0:
            print("-------------------")  #seperates grids uniformly
        for j in range(len(bo[0])):
            if j % 3 == 0 and j != 0:
                print("|",end="")
            if j == 8:
                print(bo[i][j])
            else:
                print(str(bo[i][j]) + " ",end="")

def find_empty(bo):  #empty squares with value as 0
    for i in range(len(bo)):
        for j in range(len(bo[0])):
            if bo[i][j] == 0:
                return (i , j) #row,col
    return None

print_board(board)
solve(board)
print("-------------------")
print_board(board)