编写一个字符界面的五子棋人机对战程序，游戏难度中等

为了实现这个五子棋人机对战程序，我们可以分为两个部分：一是游戏逻辑的实现，包括棋盘的绘制、下棋的判断和胜负的判断；二是人机对战的实现，包括人的输入和机器的反应。

以下是一个可能的实现：

import os

# 定义棋盘大小和棋子的类型
BOARD_SIZE = 15
PLAYER_CHESS = 'X'
AI_CHESS = 'O'


# 初始化棋盘
def init_board():
    board = [['+' for y in range(BOARD_SIZE)] for x in range(BOARD_SIZE)]
    return board


# 绘制棋盘
def draw_board(board):
    os.system('cls' if os.name == 'nt' else 'clear')  # 清屏操作
    print('  ', end='')
    for x in range(BOARD_SIZE):
        print(chr(x + ord('A')), end=' ')
    print()
    for i in range(BOARD_SIZE):
        print('{:2d}'.format(i+1), end=' ')
        for j in range(BOARD_SIZE):
            print(board[i][j], end=' ')
        print()


# 判断胜负
def check_win(board):
    # 判断行
    for i in range(BOARD_SIZE):
        for j in range(BOARD_SIZE - 4):
            if board[i][j] == board[i][j + 1] == board[i][j + 2] == board[i][j + 3] == board[i][j + 4] and board[i][j] != '+':
                return board[i][j]

    # 判断列
    for i in range(BOARD_SIZE - 4):
        for j in range(BOARD_SIZE):
            if board[i][j] == board[i + 1][j] == board[i + 2][j] == board[i + 3][j] == board[i + 4][j] and board[i][j] != '+':
                return board[i][j]

    # 判断正对角线
    for i in range(BOARD_SIZE - 4):
        for j in range(BOARD_SIZE - 4):
            if board[i][j] == board[i + 1][j + 1] == board[i + 2][j + 2] == board[i + 3][j + 3] == board[i + 4][j + 4] and board[i][j] != '+':
                return board[i][j]

    # 判断反对角线
    for i in range(BOARD_SIZE - 4):
        for j in range(4, BOARD_SIZE):
            if board[i][j] == board[i + 1][j - 1] == board[i + 2][j - 2] == board[i + 3][j - 3] == board[i + 4][j - 4] and board[i][j] != '+':
                return board[i][j]

    # 没有胜者
    return None


# 判断当前位置是否可以下棋
def can_put_chess(board, x, y):
    return board[x][y] == '+'


# 人下棋
def player_put_chess(board):
    while True:
        input_str = input('请输入落子位置(A1-O15): ')
        try:
            x = ord(input_str[0].upper()) - ord('A')
            y = int(input_str[1:]) - 1
            if x < 0 or x >= BOARD_SIZE or y < 0 or y >= BOARD_SIZE:
                print('输入位置不合法，请重新输入！')
                continue
            if not can_put_chess(board, x, y):
                print('当前位置已有棋子，请重新输入！')
                continue
            board[x][y] = PLAYER_CHESS
            return
        except Exception:
            print('输入格式不合法，请重新输入！')


# 机器下棋
def ai_put_chess(board):
    # 简单的AI：随机下棋
    import random
    while True:
        x = random.randint(0, BOARD_SIZE-1)
        y = random.randint(0, BOARD_SIZE-1)
        if can_put_chess(board, x, y):
            board[x][y] = AI_CHESS
            return


# 主函数，游戏入口
def main():
    board = init_board()
    draw_board(board)
    turn = 0
    while True:
        if turn % 2 == 0:
            player_put_chess(board)
        else:
            ai_put_chess(board)
        draw_board(board)
        winner = check_win(board)
        if winner is not None:
            if winner == PLAYER_CHESS:
                print('恭喜你获胜！')
            else:
                print('很遗憾，你输了。')
            break
        turn += 1


if __name__ == '__main__':
    main()

这个程序的具体实现可能会有细微的差别，但是大体上是这样的。其中，`os.system('cls' if os.name == 'nt' else 'clear')`这一行是为了在不同的操作系统下清屏，因为Windows和Linux的清屏命令不一样。`check_win()`函数使用了一些嵌套的循环来判断胜负，这也是五子棋程序实现的难点之一。对于AI的实现，这里只是使用了一个简单的随机下棋的做法，实际上还可以使用更为复杂的算法来提高游戏难度。