Sudoku Solver in Python Script

def find_next_empty(puzzle):

    # finds the next,row,col on the puzzle that's not filled yet --> rep with -1
    # return row,col tuple(or (none,none) ) if there is None
    # keep in mind that we are using 0-8 for our indices

    for r in range(9):
        for c in range(9):
            if puzzle[r][c] == -1:
                return r,c

    return None, None # if no space left


def is_valid(puzzle,guess,row,col):
    # figures out whether the guess at the row/col is valid guess
    # returns True if valid, returns False otherwise

    row_vals = puzzle[row]

    if guess in row_vals:
        return False

    #columns

    col_vals=[puzzle[i][col] for i in range(9)]

    if guess in col_vals:
        return False

    # now the squares
    # I want to get 3x3 square starts
    # and iterate over the 3 values in the row/column

    row_start = (row//3)*3 #1//3 = 0 throw away the remainder
    col_start = (col//3)*3

    for r in range(row_start,row_start+3):
        for c in range(col_start,col_start+3):
            if puzzle[r][c] == guess:
                return False

    #return this if it reach here

    return True



def solve_sudoku(puzzle):
    # solve sudoku using backtracking
    # our puzzle is a list of list
    # where each inner list is a row in our sudoku puzzle
    # return whether a solution exists
    # mutates puzzle to be solution (if solution exists)

    # step -1 choose somewhere on the puzzle to make a guess

    row, col = find_next_empty(puzzle)

    # step 1.1 if there's nowhere left, then we're done because we only allowed valid inputs

    if row is None:
        return True

    #step 2: if there is a place to put a number, then make a guess between 1 and 9

    for guess in range(1,10):
        #step 3: check if this is a valid guess
        #step 3.1: if this is valid then place that guess on the square

        if is_valid(puzzle,guess,row,col):
            puzzle[row][col]=guess

            #now recurse using this

            #step4 recursively call our function

            if solve_sudoku(puzzle):
                return True

        # step 5 if not valid or if our guess does not solve the puzzle

        puzzle[row][col]=-1 #reset the value

    # step 6 if none of the numbers we try didn't work then puzzle is unsolvable

    return False


if __name__=="__main__":
    example_board=[
        [3,9,-1, -1,5,-1, -1,-1,-1],
        [-1,-1,-1, 2,-1,-1, -1,-1,5],
        [-1,-1,-1, 7,1,9, -1,8,-1],

        [-1,5,-1, -1,6,8, -1,-1,-1],
        [2,-1,6, -1,-1,3, -1,-1,-1],
        [-1,-1,-1, -1,-1,-1, -1,-1,4],

        [5,-1,-1, -1,-1,-1, -1,-1,-1],
        [6,7,-1, 1,-1, 5, -1,4,-1],
        [1,-1,9, -1,-1,-1,  2,-1,-1]
    ]

    print(solve_sudoku(example_board))
    print(example_board)