Tic Tac Toe AI | Python Fiddle

X_SCORE = 10
O_SCORE = -10
BLANK_SCORE = 1

class Node:
    def __init__(self, value):
        self.value = value
        self.score = -1

class Counter:
    def __init__(self):
        self.score = 0

def add(self, value):
        if value == 'x':
            self.score += X_SCORE
        elif value == 'o':
            self.score += O_SCORE
        elif value == '_':
            self.score += BLANK_SCORE
        else:
            raise ValueError(value)

def get_score(self):
        return abs(self.score)

class Board:
    def __init__(self, l=None):
        nodes = {}
        row_counter = [Counter() for _ in xrange(len(l))]
        column_counter = [Counter() for _ in xrange(len(l[0]))]
        diagonal_counter_right, diagonal_counter_left = Counter(), Counter()

for row_num, row in enumerate(l):
            for column_num, value in enumerate(row):
                nodes[(row_num, column_num)] = Node(value)
                row_counter[row_num].add(value)
                column_counter[column_num].add(value)
                if row_num == column_num:
                    diagonal_counter_right.add(value)
                if row_num + column_num == len(l) - 1:
                    diagonal_counter_left.add(value)

for row_num in xrange(len(l)):
            row_count = row_counter[row_num]
            for column_num in xrange(len(l[0])):
                if l[row_num][column_num] != '_':
                    continue
                column_count = column_counter[column_num]
                score = 0
                score += row_count.get_score() + column_count.get_score()
                if row_num == column_num:
                    score += diagonal_counter_right.get_score()
                if row_num + column_num == len(l) - 1:
                    score += diagonal_counter_left.get_score()
                nodes[(row_num, column_num)].score = score

self.nodes = nodes

def to_output(self):
        nodes = self.nodes
        board = ''
        for row_num in xrange(3):
            board += '|'.join([nodes[(row_num, column_num)].value for column_num in xrange(3)]) + '\n'
        return board

def from_input():
    return Board([raw_input().split('|') for _ in xrange(3)])

def main():
    while True:
        board = from_input()

best_node = None
        best_node_score = -1

for k, v in board.nodes.iteritems():
            if v.score > best_node_score:
                best_node = k
                best_node_score = v.score

if not best_node:
            break

board.nodes[best_node].value = 'x'
        print board.to_output()

if __name__ == '__main__':
    main()

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