main Function

Next, let's code the main function of the game and two helper functions called get_moveand print_mat

Below is the complete functional code for this phase of the game

# Main function: alternately prompt the two human
# players – 'X' and 'O' – by calling get_move function.
def main():
    num_moves = 0
    print_mat()
    print("Moves are r, c or 0 to exit.")
    exit_flag = False
    while not exit_flag:
        num_moves += 1
        if num_moves > 9:
            print("No more space left.")
            break
        if num_moves % 2 > 0:
            player_ch = 'X'
        else:
            player_ch = 'O'
        exit_flag, r, c = get_move(player_ch)

# Get Move function.
# Prompt and re-prompt human player ('X' or 'O')
# until a valid move of form 'row, col' has been
# entered at an available square. Then enter move
# into the grid and re-print the grid display.
def get_move(player_ch):
    while True:
        prompt = 'Enter move for ' + player_ch + ': '
        s = input(prompt)
        a_list = s.split(',')
        if len(a_list) >= 1 and int(a_list[0]) == 0:
            print('Bye now.')
            return True,0,0   # Throw 'EXIT' flag
        elif len(a_list) < 2:
            print('Use row, col. Re-enter.')
        else:
            # First, convert to 0-based indexes.
            r = int(a_list[0]) - 1
            c = int(a_list[1]) - 1
            if r < 0 or r >= n or c < 0 or c >= n:
                print('Out of range. Re-enter.')
            elif mat[r][c] != '.':
                print('Occupied square. Re-enter.')
            else:
                mat[r][c] = player_ch
                print_mat()
                break
    return False, r, c # Do not throw 'EXIT' flag

def print_mat():
    s = ' | 1 | 2 | 3 |\n'
    s += '-------------\n'
    for i in range(n):
        s += str(i + 1) + '| '
        for j in range(n):
            s += str(mat[i][j]) + ' | '
        s += '\n'
    print(s)
##############################################
#main part of the code
n = 3
mat = [['.', '.', '.'],
       ['.', '.', '.'],
       ['.', '.', '.']]
main()

Let's explain the above code in more details. The main function (lines 4 - 15). The main function alternates between X and O using the modulus operator %. Even number of moves, means it's the 'O' turn and odd number of moves, means it's the 'X' turn. The main function then calls the get_move function.

The get_move function returns True if and only if the user types 0 to terminate the game early. When that happens, the exit_flag variable in the main function is switched on (set to True) within the main function, and the program terminates (the while not exit_flag will evaluate to False). The main function also terminates the game if nine moves have been made (notice the break command in line 13).