def check_diagonal_winner(input_list, size):
    """
    Check the winner number in diagonal direction.

    Arguments:
    input_list -- a two dimensional list for checking.
    size -- the length for winning.
    
    Returns:
    winner -- the winner player number, if no winner return None.
    
    """
    for row_idx, line in enumerate(input_list):
        winner = ' '
        try:
            list_for_check = []
            for i in range(size):
                list_for_check.append(input_list[row_idx+i][i])
            if list_for_check.count(list_for_check[0]) == size:
                if list_for_check[0] != ' ':
                    return list_for_check[0] 
        except IndexError:
            winner = ' '