def find_next_empty(puzzle):
    """
    Takes a puzzle as parameter
    Finds the next row, col on the puzzle
    that's not filled yet (with a value of -1)
    Returns (row, col) tuple
    or (None, None) if there is none
    """
    for row in range(9):
        for col in range(9):
            if puzzle[row][col] == -1:
                return row, col

    return None, None